What reason would the IRS have for saying my return does not match my W2s?

[VolvoGirl]

They might not have got a copy from your employer to match.  Double check your entries.  Does 1040 line 1 and line 16 match your W2s? You might have entered a box wrong.

[Carl]

You also may have used W-2's for the wrong year. For example, if you filed a 2018 tax return, is it possible you used W-2's from 2017? So check that, as that mistake is not as uncommon as one may think.