I'll take a crack at a possible explanation based on what I remember from an answer Knox gave me a while back about this. My numbers are estimates.
This happens whenever posts are being submitted in rapid succession on forums that you have excluded in your spy preferences. I think by default, without any exclusions, forum spy displays 50-ish posts or 15 minutes worth, whichever comes first. When loads of posts are being submitted in your excluded forums those 50-ish posts are merely being hidden, so you're only seeing a few minutes worth of posts from forums you have INcluded.
I'm hoping that makes sense/is correct.
I noticed it yesterday too. I laughed when I only had 4 posts displayed in forum spy. I only exclude about 15 forums, your 56 made me giggle.
When the forums are positively buzzing with posts, such as on update days, keynotes, etc, I think it's typically better to just enter the specific forums you're interested in rather than rely on the spy.