I know this sounds dumb, but are they somehow lighter? Is it the optical illusion of seeing a huge plate that only weighs X?
My gym just got a real platform and bumper plates, and I was deadlifting today. I worked my way up and when I got to (too little weight to mention here) it seemed like a lot less than usual. I was able to do more reps with it, and then add quarters and do another set. I'm certain that I didn't get that much stronger that quickly, and would like a little perspective.