GMail spam count

A few months ago, I decided to not to delete spam mesages, just because I can keep them all thanks to Gooooogle ;-). And than I noticed something very interesting with my GMail spam count. At first, I thought that I didn't remember the correct number, but than I decided to make a screenshot each day at the same time. The result is a little strange...

The number of spam e-mails is bigger one day, and smaller the next day. WTF? I cannot explain this, but it's freaking me out, man! ;-)