Tracking Error

The new S&P 500 Index Fund was supposed to track ticker SPY. But it didn't, The algorithm that the new SYPX used was a bit different from SPY (the most popular and largest index fund, as managed by Vanguard). SYPX reinvested dividends quarterly, whereas SPY...didn't. They just paid out the dividends to investors. SYPX had it as default that those divvys would be reinvested. As a result, the two funds didn't "track," or follow, the same number pattern on the fancy results charts they produced, marketing to investors.

Over time, the error compounded, and oh so not surprisingly, SYPX did a whole lot better than SPY, because more capital was put to work buying more shares. At least while the bull market remained.