The stock market's identical twins are looking like opposites.

You might think market-tracking funds that track the same part of the market would produce the same returns. They don't.

Just look at exchange-traded funds that invest in large, fast-growing U.S. companies. Sure, they all hold lots of technology and AI-related stocks, such as Alphabet or Nvidia. But their recent returns are drastically different.

So far in 2026, the performance gap across large-cap growth index ETFs from Invesco, iShares, State Street and Vanguard is staggering. The iShares Russell 1000 Growth ETF is up 4.6%, while Invesco S&P 500 Pure Growth has gained 24.3%. That gap isn't unique to big growth stocks. The difference between the top and bottom performer among leading value ETFs, which invest in indexes of big, cheaper U.S. companies, is 11 percentage points this year.

For investors, this is a reminder of two basic principles: You can't judge a fund by its name alone, and diverging even slightly from the broadest exposure to stocks can give you startlingly different results.

Professional investors have long segmented the market by how large and how fast-growing stocks are, although the definitions of "large," "small, " "growth" and "value" are all over the map. One index's large value stock is another's midsize growth stock. Indexes also tweak their lists of constituents on different schedules.

There's been endless talk in the past few years about how the market has become overconcentrated in a few giant companies. Another point gets less attention: The finer you slice-and-dice the stock market, the more concentrated it becomes.

Owning a large-growth or large-value index -- instead of a fund that holds the entire market -- reduces your diversification and raises the risk that a handful of stocks can create wacky short-term results.

The biggest differences this year across similar-sounding index funds aren't coming from variations in their exposure to market sectors or industries, says Matthew Bartolini, global head of research strategists at State Street Investment Management. They're coming from a few stocks -- very few.

Consider Micron Technology and Sandisk. These two memory-chip makers have been soaring on the back of the AI boom.

But, until the end of June, the two were in the Russell 1000 Value index. At that point, they moved into the Growth index.

Why the value categorization? In Russell's definition of value, the price-to-book ratio -- a measure of stock price relative to net assets -- looms large.

Chip makers often own equipment, factories and other assets that contribute significantly to book value. That helped these stocks score relatively well on that measure.

So the stocks' scorching performance this year -- as of July 31, Micron was up 188.6% and Sandisk 411.8% -- contributed a fifth of the total return of the Russell 1000 Value index in that period, says Catherine Yoshimoto, director of product management at FTSE Russell.

That largely explains why the Russell 1000 Value index was up 20.7% this year through the end of July, while its sibling growth index gained a paltry 0.3%.

As a result, ETFs tracking Russell's large-stock growth benchmark, such as iShares Russell 1000 Growth, trailed far behind funds linked to other large-growth indexes with bigger positions in hot chip stocks. On the other hand, large value funds tied to other benchmarks, which had lower exposure to the hottest memory stocks, lagged behind iShares Russell 1000 Value.

Even in index funds whose names sound similar, "people should have the expectation that different definitions will end in different results," says Jay Jacobs, U.S. head of equity ETFs at BlackRock, which runs the iShares funds.

Invesco S&P 500 Pure Growth holds only a few dozen stocks that score highest on long-term growth and medium-term positive price change.

At the index's annual adjustment in December, Advanced Micro Devices, Micron and other chip makers met the standards for inclusion. The Pure Growth ETF added them -- and they kept soaring. The fund is up 24.3% this year, far ahead of other large-growth index funds. In 2025, before the latest annual change, it trailed other large-growth index ETFs by at least 5 percentage points.

The fund's high return this year is "a perfect storm of the criteria to establish a growth name as well as the timing of when the reconstitution period happened," says Rene Reyna, head of equity ETF product strategy at Invesco.

Do you even need to segment your portfolio into growth and value? Splitting the market in two and owning both halves is a lot like owning the whole thing in the first place.

A roughly equal mix of funds tracking the Russell 1000 Growth and 1000 Value indexes, for example, should approximate the return of the Russell 1000 over time.

But all bets are off if you use growth and value funds that track indexes from different providers. In my experience, many financial advisers do this sort of grazing, picking a growth fund from one manager and a value fund from another.

That's a bad idea. The variation in standards and schedules across different index providers means you could end up with a patchwork.

"Don't mix and match," warns Kathy Kellert, who heads stock-index strategy at Vanguard. "I think that's the most common pitfall we see. You can get overlaps in exposures, or gaps in exposures, that you don't realize that you have."

Advisers are also fond of what they call "tilting," favoring growth or value based on their view of which style seems more attractive at the moment.

That's an example of what I've called "deversification" -- moving away from broad diversification to concentrating on a narrower segment of the market. It makes your investing results less predictable.

Even something that sounds simple and straightforward in the long run, like "large growth" or "large value," can be a roll of the dice in the short run.