Even with good guide bases, I frequently find slots cut in this way to be inconsistent, usually slightly wider at the ends where the cutter is slowed down and reversed. That in addition to the fact that a 1/8' or 3/32' shank on a cutter just can't handle much of a load without flexing can lead to a lot of slop in the cut.
I don't know about the Rotozip, but I can't help but imagine that it's bearings are not much better or more solidly anchored than those on a Dremel.
One is simply the tolerances of the tools you're using.
