Two identical heatsinks may NOT perform the same way.

Besides the mounting issues with some TRUE's or lapping, etc, I had two XP-90's years ago, and the second XP-90 had 5C *lower* temps than the first one. And it had nothing to do with mounting (i probably remounted those things over 10 times), although I think I used AS2 and later 3 as AS5 wasn't out yet.
Explanation from someone (here or ocforums) said it was probably a faulty heatpipe. If that's the case, than it's easily possible that this can happen with TRUEs and other heatsinks as well.

I had come from a SLK-947U (i forgot..was this copper?) and I remember the "bad" XP90 giving lower temps (5c?) over that...