I would think that when using an incubator - that any temperature inaccuracies would make a difference. Mine was running cool and I would get late hatches 22-23 days. In that same incubator - I once had a youngster hatch before I moved them to the hatcher - day 19. Go figure. A broody would be able to give the eggs ideal conditions temp-wise.That is what I had heard also so I was expecting a slightly early hatch but all my Silkies for that hatch hatched Jan 1st-3rd(day21-23). Maybe it is because they were shipped Im not sure.
My Japanese Bantams hatch typically on the 20th and 21st day. Ive only gotten day19 hatchers from under a broody.