Fertile egg query

Hi folks. I know I've posted quite a bit on here lately, but there's always a mine of information available if you have a query! I have 7 eggs in my incubator and have 7 days to go before hatching. When I candled the eggs last night, one egg seems more advanced than the others. It's definately fertile and not an "exploding" egg, but the egg seems a lot fuller than the others. I have 5 Australorp eggs and 2 of my own fertlie eggs which I took after my broody rejected them. The broody has been sitting on the 2 eggs half a day less than the incubator ones, but it is one of my own eggs that seems more developed. I'm absolutely certain of the dates my eggs went under the broody, so l know they have not been brooded longer. Is it possible that eggs under a broody develop faster or can eggs hatch before the 21 day deadline?

Thanks Hillfooter. I usually only candle twice, but because this egg even at the early stage looked more developed l thought l'd give it one more candle. My incubator is automatic so will stop turning after day 18 anyway.
I do have an Orpington cockerel running with my ladies (as well as a Welsumer), but thought he may be a bit young yet to tread the ladies. It may be I have a giant chick!