they are definitely inconsistent. I think it's cause they are two-piece jobs with the base/fan holder vs the fins. mine definitely was concave. add that to the fact that the GeForce 256 is concave as well and you've got quite the gap.
I spread a decent layer of ASII on the chip (knowing it was concave) and put on the BLORB. I like to get a nice suction between the HSF & chip, but couldn't do it. Removed the orb and looked at the base. A nice, uneven, empty circle of ASII greeted me. No contact in the center of the orb!
lapping the orb definitely improved both the contact & the temps I was getting (the Asus board has HW monitoring). Just for ha-has I ran the board prior to lapping. the temps were higher than with the retail HSF/silicone goop by about 4 degrees! now w/ lapping got about 3 degrees lower. still not great performance (IMHO), but it's cooler so i'm keeping it!