Do you mark spam email that you get as spam? I rarely get spam in my GMail, but when I do I mark it as spam. Of course, maybe our habits are just different, but I like to reassure myself that I'm helping the system chug along.
I do, but only on the occasional times I check Gmail at all. It's possible it'd become better for me with more intense use/training, but I'm not ready to leap into even more dependency on Google services based on the experience so far.
Ah, well in that case it's probably partially explained by a lower volume of legitimate email that you get there if you don't really use it much.