If everyone takes a straight-line path between dots and does it with a consistent step-size the entire way, the drill should, in theory, be perfectly clean.
It's more difficult than that in practice, but that's what mid-sets and quarter-sets are for. They're very useful and any dot-oriented corps is going to make sure their members have all of their sets and mid-sets memorized.
Because marching members are human. There's always going to be some variation in performance quality.
In the dot system as practiced by the Cavaliers and SCV, marchers do not pay attention to those around them. They're using their eyes constantly to check in with the drum major and to check field references (hashes, numbers, yardlines, tick marks, and any other markings that may exist).