If you're a leaf, you're sheilded (by ultrapeers) from unnecessary traffic & you'll only see incoming searches that relate to your files. Whereas an ultrapeer will see heaps more searches as they're using their bandwidth for handling more messages & searches.
http://www.gnutellaforums.com/showth...oming+searches

People who aren't firewalled & have a fast connection will run as an ultrapeer when there's a need for ultrapeers...the network decides