The question just about says it all. See the attached chart, in which VOO is darker blue, the index is light blue, and SPY is violet - leading up to June 28th, the 3 moved more or less in lockstep, as I would expect. From June 28th on, the ETF has been consistently below the other two. As I write this, the index is up .01%, SPY is down .01%, but VOO is down .17%. Why?
VOO paid a dividend of 1.157 on July 3rd for anyone holding shares as of June 28th, so effectively the "value" of the ETF went down by the same amount (since part of its assets were distributed as cash.
The S&P 500 Index includes dividends that were paid, hence the difference.
I suspect you'd see a similar difference with SPY if you chart back before its last dividend on June 15th.
The total return (price return plus dividends) should track the S&P, not just the price itself.