I think that this is a fairly modern problem. You'll notice if you look at your older CDs, the CD is slightly thicker and there's usually a bead that runs around the center hub. This is probably the extra layer of plastic on top. Modern CDs don't have that bead, which may or may not imply that that extra layer of plastic is missing. I think the reason that it happened was to make printing on the CDs easier. You'll notice that many CDs these days have printing right up to the hole, whereas earlier ones only had it up to the bead, except for a few that they probably knew were going to be hits, and they have a gap over the bead and then more printing inside.

All of this is just conjecture, though. And probably not correct, at that. It's probably just my New Coke conspiracy theory all over again.