I really think people aren't actually considering the actual centers in the league when they think of KG at the 5 95% of the time. And the other 5% of the time, he'll do at least as well as other 5s against the likes of Bynum and Howard.
Or how difficult it'll be for centers to defend Garnett on the other end of the floor.
Few fives are comfortable stepping out 17-20 feet away from the basket which they'll have to because of David West's + Kevin Garnett's jump-shooting ability. There is nobody they can hide their five on while they're in the game together. And I mean this not only in terms of man-to-man defense but knowing how to play effective team defense.
Most fives do not know how to rotate and recover properly against highly skilled centers who can play away from the basket. They really struggle to know when to commit to the help D and when to stay at home. Regularly get caught out in no-mans land.
It's the balance of the matchup overall, in terms of offense + defense + rebounding, that matters.
Not solely defense and not solely man-to-man defense (about 25-30% of a center's defensive responsibility) and not solely post defense (which apparently only accounts for about a third of a center's man-to-man D). Have to look at everything. Not get too bogged down in one area and lose focus of the whole picture.
Kevin Garnett is a very effective center.