Are you talking about absolute price or daily percent change? The daily percentage change should be close to identical. Price over time drifts apart because IVV can lend shares and reinvests dividends before payout where as SPY does not. So they're not tracking exactly the same thing, i.e. right before dividend payout IVV actually holds more of the index than SPY. There is also a difference in fee structure between the two, although I think that's a second order effect compared to the other two differences.