Probably been mentioned before somewhere here, but it's a bug - large multi-folder filesets, when initialised, show the wrong percentage done value as they are being written out to disk (full writes, not sparse). The percentage value stays at 0 until after the final folder in the fileset is reached, and only then starts increasing.

This means the total initialised content is somehow not being calculated correctly as the files are individually written, which looks weird and is very disconcerting for big filesets. Maybe this can be looked at. (Currently using 2.87)