Two Index Funds Tracked the Same Market and Ended $18,400 Apart

The number on my laptop screen at 6:15 AM on a Thursday was $18,412, and I woke my husband up to show it to him because I was sure I had made a spreadsheet error. Kitchen still dark, coffee not made, two fund fact sheets open side by side, and a gap between them that should not have existed. Same index. Same market. Nine years of holding. One of my funds had simply, quietly, fallen behind the thing it promised to copy.

He looked at it, said "that can't be right," and went back to sleep. It was right.

the assumption I never tested

Back in 2017 I split a $250,000 inheritance between two broad index funds, mostly cuz they were offered at two different brokers and I wanted both signup bonuses. Everyone I trusted said index funds are commodities, that the ticker matters less than the fact you own the market. Expense ratios were 0.03% and 0.09%. Six hundredths of a percent, I remember thinking, that is rounding error. That figured cost me the price of a car.

what tracking error actually does

Here is the mechanic nobody explains at dinner parties. An index fund does not own the index. It owns a sample, or it owns everything, and it pays trading costs, suffers cash drag from dividend timing, and occasionally lends out securities or does not. All of that shows up as charting difference, the gap between what the index returned and what the fund returned. The S&P 500 as a number does not pay fees. My fund did, in ways beyond the headline expense ratio.

One of my funds compounded at essentially the index, off by 2 basis points a year. The other lagged by 19. Over one year that is invisible. Over nine years of compounding on a quarter million dollars, it becomes $18,412, which I now know to the dollar.

the chart that hid in plain sight

The odd thing is I checked performance constantly. I was that person who opened the brokerage app too frequently. But I invariably compared my funds to each other and to headlines, never to the index itself. The lagging fund still showed a lovely upward line. Up and to the right hides a lot of sins. A fund can underperform its benchmark every single year for a decade and never once look like a mistake on a client portal.

why I did not sell immediately

My first instinct was to swap everything into the tighter fund. My second instinct was to read the tax form. The lagging fund sat in my taxable account with $91,000 of embedded gains. Selling meant writing a capital gains tax check that year of approximately $13,000 at my rates. The charting gap was real, but it accumulated at about $2,000 a year, while the tax hit would be front-loaded.

So the arithmetic said something counterintuitive. In the taxable account, stay put, direct every new contribution to the better fund, and let the gap grow slower than the tax penalty of fixing it. In my IRA, where selling is free, I moved the whole position the same week.

the fee discussion I finally had

I called the lagging fund's shareholder services and asked directly why the logging difference ran 19 basis points when the expense ratio was 9. The rep read me a scripted answer about market conditions. I asked for the fund's own published logging report, which exists and which almost nobody requests. Securities lending revenue, sampling error, cash positions between dividend dates. It all added up on paper exactly the way my spreadsheet said. They were polite. They did not dispute it. There was nothing to dispute, the gap was disclosed in ten-point font on page 14 of a PDF.

what I tell people now

I used to say picking an index fund is like picking water. I don't say that anymore. Same label, same index, and one of my choices leaked $2,000 a year thru a hole you can only find by comparing the fund to its own benchmark, fund against fund tells you nothing. When friends ask me now which index fund to buy, I give em an odd answer than they wanted. Pick three candidates, pull each fund's cumulative return versus its index over ten years, and buy the one that hugs the line. Then never let the tax tail wag the dog when you fix old mistakes.

the second spreadsheet, a year later

It has been fourteen months since that dark kitchen. The new contributions have all gone into the tight tracker, and my blended logging difference across the household has dropped from 11 basis points to 4. On this morning run of the same spreadsheet, the projected ten-year gap between what I hold now and what I would have held under the old split is down to $6,900 and shrinking. The number $18,412 is still in cell F7 where I first saw it. I never deleted it. It is doing a different job now, not a wound, a receipt.