My recollection from military aircraft in which I wore an oxygen mask is that we used 100% when there was considered a risk of contamination or (in a few types) during engine start when there was considered a risk of sudden cabin air contamination.
Given that below FL250 cabin alt, it should be mixing progressive amounts of cabin air and O2, using "normal" should mean that the oxygen supply, which presumably is limited with an emergency system (unless you have an OBOGS which seems unlikely in an airliner) will last as long as possible. This I'd regard as a good thing.
G