That's the nature of film.
Physical things like lint, hair and dust can affect certain sections of the print, then get swept
away as the frames chug by.
Sometimes, if the dirt is gritty, it can permanently scratch the film or tightly imbed itself
on the print.
Certain of these artifacts (in combination) can make each film performance look slightly different
as the various bits dust, lint, hair, and permanent scratches appear and disappear
as the print chugs through the projection gate.
The easiest (and most expensive) solution is to thread up a new print (copy of the film)
as it will not have any (or many) of these physical distractions present.
Again, Disney may keep some pristine copies of the print ready to thread up for
"VIP" performances of the show(s.)
.