That's not the reason for the disparity. I, for example, actually have more posts logged than my "official" count indicates.
I think there are just hiccups in the software where an operation's database operations don't ALL complete and so you may be left in a situation where the post count is updated but the actual post doesn't go through for some reason, or a post is deleted but the count fails to be updated, and while this is rare, they can accumulate over time.
As for me, I actually see a different number of posts for him as my version of the post history shows reported posts (posts he reported). It's just a byproduct of how the report system works. Reporting a post does increase your post count by 1, for that reason.
Perhaps one of these days I should have the forum re-calculate actual post counts 
Looking through, it looks like most of the most prolific posters have slightly overstated counts, but many posters with post counts like mine actually have slightly understated post counts. I have a few more listed on my profile than my official number (9833 vs. 9826). Do you know why it works differently for less prolific posters?
Poll
