A few reasons. 1. You drop of the front page of the category you're in, so any traffic that feeds back to that page won't find your article. 2. When Google catches the page, you probably make it into the top 10 for a few days. Than Google sends it off somewhere else for a few weeks. Sometimes it comes back to the top 10, other times it doesn't.
Yup - that can be a bugger. Also - the theory that an article sits on the pile - or the front page over the weekend and gets loads more hits, I think is no longer applicable as I'm now having articles accepted at the weekend - but their staff might be lighter.
I've noticed the power of ezinearticles has gone down drastically since I started using it and that's not for a very long time. It's sad in a way.