I this related to when you are scrolling and selecting within a document, and you wiggle the mouse, it scrolls faster ? I always thought it was just a nice UI optimisation, but I could believe it's actually some accidental side-effect at play.

(like make a 20 page word doc, and start selecting from the first page and drag through - it wil go faster if you jiggle. same in excel and nearly every windows app, even windows explorer)

No, it has to do with every time you move the mouse over a window, a hover event is sent to the application, which runs its main event loop. Either the installer only updated its progress bar when an event happened (in which case it would only appear to be going faster, because the progress bar would move more smoothly) or there was some really terribly written code that literally only made progress when an (unrelated) event happened. My guess is the former.