On what basis does the Finder update the file copy progression bar?
To make a somewhat educated guess


The bar shows "time spent" vs. "overall time expected"
"overall time expected" is somewhat dynamic, depending on current throughput. So if you start of with a lot of small files (which usually take longer than a large one) transfer seems to speed up later (and vice versa)
I'm not aware of any way to change this


PS: And no, I don't have a reference for that, just years of observation