One thing that always catches people out are completions.

Passing completions only show who threw and who caught but not who tackled. But he only made one completion your last game, so I'm not so sure how much that makes up for 'off' tackling numbers.

EDIT: I just counted all the tackles I thought your defence would have put on his offence and got 30, but only see 27 in your stats (though I may have counted ingame wrong). Mind you, I've never really paid close attention to this, so I'm not sure what's the answer here!

i think the OP does not have any passing stats in the calculation for Number of tackles.
Logically that calculation seems right - So if you used the box score tab and added(or subtracted ) those stats, they should add up to the total number of tackles

That said, I never pay too much attention to the number of tackles, etc .

Someone else maybe able to answer