You picked the “cheap” S&P 500 ETF over the expensive one. Smart move, right?
Maybe not. Two ETFs tracking the exact same index can produce noticeably different returns over time, sometimes by a full percentage point or more. And expense ratios are only part of the story. If you’ve ever wondered why VOO and SPY don’t move in perfect lockstep, here’s what’s actually going on under the hood.
The Number That Misleads Most Investors
Expense ratios get all the attention. VOO charges 0.03% per year. SPY charges 0.0945%. On a $50,000 investment, that’s roughly $15 versus $47 annually. VOO wins, case closed.
Except it isn’t.
The real measure of an ETF’s efficiency is something called tracking difference: the gap between what the index actually returned and what the ETF actually returned over the same period. An ETF with a 0.10% expense ratio could theoretically outperform an ETF with a 0.03% expense ratio if it’s managed more efficiently. Yes, that’s possible. No, most investors don’t know it.
Tracking difference is the final score. Expense ratio is just one player on the team.
Why Dividends Are More Complicated Than They Look
Here’s a scenario. The S&P 500 index assumes that every dividend paid by its 500 companies gets reinvested immediately and perfectly on the ex-dividend date. No delays, no friction, no taxes. Just instant, frictionless reinvestment.
Real ETFs don’t work that way.
When Apple or ExxonMobil pays a dividend, the ETF collects that cash. But there’s a lag, sometimes days, before that cash gets reinvested into the fund. During that window, the market might move up. The fund holds cash instead of stocks, misses the gain, and falls slightly behind the index it’s supposed to mirror.
This drag is called dividend reinvestment timing, and it’s why even two ETFs with the same expense ratio can diverge. The fund that processes dividends faster, or that structures its cash management more efficiently, earns slightly better returns over time. Small gaps compound into real money over a decade.
The Secret Revenue Stream Helping Some ETFs Beat the Index
This one surprises people. Some ETFs actually outperform their benchmark index after fees. How?
Securities lending.
ETFs can loan out shares of the stocks they hold to short sellers who need to borrow them. The short sellers pay a fee for that privilege. The ETF collects the fee, keeps a portion, and passes the rest back to shareholders. It’s a real, recurring income source that can partially or fully offset the fund’s expense ratio.
Vanguard’s fund structure is particularly aggressive here. Because Vanguard funds are owned by the fund shareholders themselves (not an outside corporation), profits from securities lending flow directly back to investors. This is one reason VOO and other Vanguard funds sometimes have a tracking difference that’s actually negative, meaning they slightly outperform their stated benchmark.
SPY, managed by State Street, also lends securities, but under a different structure where the returns are shared differently. The result: two funds, same index, different bottom lines.
When “Tracking the Index” Isn’t What You Think
There’s another layer. Most people assume an S&P 500 ETF simply owns all 500 stocks in the index, weighted exactly right. That’s called full replication, and it’s what VOO and SPY generally do.
But some ETFs, particularly those tracking less liquid or more exotic indexes, use sampling. Instead of buying every single security in the index, the fund buys a representative sample of holdings that statistically mimic the index’s behavior. This cuts transaction costs but introduces a new risk: the sample might not perfectly reflect the whole.
Even among full-replication ETFs, timing matters. When the S&P 500 adds or removes a company, every ETF tracking it needs to make that trade. The fund that executes the trade most efficiently, with better timing, lower transaction costs, and smarter order routing, falls behind less. The one that fumbles even slightly pays a price in tracking error.
Tracking error (different from tracking difference) measures the volatility of that gap over time. A fund might have a low average tracking difference but huge swings around that average. That inconsistency is its own kind of risk.
What This Actually Means for Your Portfolio
So which S&P 500 ETF should you own?
Look up the fund’s trailing tracking difference, not just its expense ratio. Morningstar and ETF.com both publish this data for free. For major index ETFs like VOO, IVV, and SPY, the differences are genuinely small — we’re talking hundredths of a percentage point on most years.
But the principle scales. When you move beyond vanilla S&P 500 funds into bond ETFs, international funds, sector ETFs, or factor funds, tracking difference can widen significantly. A small-cap ETF with a 0.25% expense ratio might have a tracking difference of 0.60% once dividend timing, sampling, and rebalancing costs are factored in. Now the math matters a lot more.
The expense ratio tells you what the fund charges you. The tracking difference tells you what the fund actually costs you.
Those are two very different numbers, and only one of them shows up in the marketing materials.
