65,535=2^16-1, the highest number that can be stored with two bytes. For some reason, the computer thinks you have -3 or -2 unread messages. The variable seems to be stored unsigned, so it gets displayed as 65,533 or 65,534.

Not sure how to get to -2 unread messages, but my Gmail on my Blackberry shows that as well sometimes when I use Gmail also on my normal computer, and it gets out of sync.