you need to look at the correct data sources. SPY and VOO both charge management fees. These fees will decrease the value of the fund and they are charged every day. The fees are small enough that you will not see them clearly versus the fluctuation from NAV.
Let's do this with VOO:
Look on Bloomberg at the ticker "VOONV Index" for the actual Net Asset Value. That comes up with a value for Aug-22-2019 of 268.43. The primary close for VOO is found under "VOO UP Equity". That value is 268.46. Right there you can see how the noise starts to filter in. Also, notice that the NAV only has two digits of precision. That's how Vanguard does it - they don't let you see more detail then that.
Now, to make it a little more fun, ETFs are actively managed trusts. That means they have cash flows from other activities. Securities lending is the most common one. iShares is a little more clear than the other trusts in documenting what goes on behind the scenes. If you look at the latest IVV annual report it looks like they get about one basis point of securities lending revenue. It's not much, but neither is the expense ratio - so they can offset each-other to a degree.
Also - ETFs hold some measure of cash as they go through their lifecycles. Some, like IVV try to minimize the cash to a basis point. Others, like SPY, can collect quite a bit of cash. This cash drag will again impact your performance vs the pure underlying index.
Lastly - what SP Index are you using for benchmarking ? The SPX ? SPTR? You meed to make sure you are comparing apples to apples. SPY targets SPX and VOO/IVV target SPTR.
** Editing to add a chart. Below is IVV NAV vs SPTR from the last IVV dividend. You can see a very slight outperformance of the index because of the management fee.
I hope that helps!