This happened to me this year. Both plants were from seeds I made last year. A cross of a Hindu Kush and a Jack Herer. I had one plant start to flower around July 1st and her sister almost 3 weeks later. Both of them well before the other plants on the property.
I assumed the difference was because I used CS on a lower branch of a Jack Herer photo period plant and pollinated a flower on a Hindu Kush autoflowering plant. My only intention was to go through the process with seeds I didn't pay for to make sure I wasn't going to screw something up when It counted lol.!
Were your seeds from a breeder or your own.?