While I'm writing this question, I've written 301 questions and 1,324 answers on this site:
Yet my network profile shows 300 questions and 1,323 answers:
I don't think it's a caching issue, as I've checked the profiles of users who posted something today and their counts have been properly updated. My newest post is ~30 hours old. Deleted posts might play a role, but my last deleted question is from last November, and none of my questions has been Roomba'ed since. My last deleted answer is somewhat newer (24 hours).
After I posted this (my 302nd) question, the number of questions on my network profile had immediately increased by 1, to 301, so it is updated but it's still off by one.
A year later, the problem still persists, but I now noticed it also happens to my MathOverflow account; the network profile says 16 answers:
one more than my site profile (15 answers):