Looking at the Olympic thread got me thinking. It seems to me that a wobbly upper/lower fit while not necessarily the most desirable, is actually an example of centered tolerances.

Here's my reasoning. In order to avoid having to individually fit each assembly, you have to allow some tolerance to assure the best interchangability. When an upper fits snugly to a lower, the average dimensions are not perfect-keeping things tight but within tolerances enough to fit. The closer to perfect the actual machine work, the looser the fit will feel because the tolerances allow the movement.
Then of course, there is just sloppy work and excess tolerance, but hey...
And of course, when things get really out of spec we read about guys hammering pins on the kitchen floor.