Lucio, as far as I can tell, the problem is with the way answers are counted for some questions.
Like you've said, that little square next to a question will often show a +1 difference between the number of answeres shown and the number of answers posted.
I've seen a lot of questions for which that box reads 1 answer, when there is actually no answer posted, or 2 answers when there is actually only 1 answer posted, even 3 answers when there were only 2 posted.
That's just a bug in how that little box "counts" the number of answers posted for some questions.
What makes the problem a little difficult to pin-point, for me at least, is that this only happens for some questions, the majority being labeled with the correct number of answers.