I find it one of the many annoyances of the digital age that on a diverse variety of devices updates or downloads often proceed at a fairly constant rate until they reach 99% (or even 100%) but then take ages to complete the last little bit.

Or the opposite. Sometimes a download says it will take say 2 hours on your internet connection , and proceed slowly, then suddenly rapidly finishes in much less time. I know this could be due to the vagaries of the internet connection, server etc and I should be grateful its quicker than the estimate , but its annoying.
A watched pot never boils

Maybe Garmin running to several hundred percent is intended to be less stressful. You never know how near it is to completion.
I always format my USB stick before downloading and have never had an update fail