Alas, there doesn't seem to be any problem with performance as a result of the uneven track pad. Aside from which, there's no way to determine the reason for the uneven trackpad surface.
This is an issue that has been mentioned in Apple Forums for over four years, now. If this bothers you - and judging from the post, it does - I'd present the case at the Apple Store's Genius Bar. Apparently, you're still not under warranty, but there's a chance that that Apple might perform the repair gratis - or quote the price. I'd document the problem with similar posts to bolster your case.
I'm interested in knowing how this comes out. Post here with the results.