Honestly the same type of thing that causes differences in seeds from the same plant. Different gene inheritance. Autos don't always auto. Sometimes you have to flip to 12/12, sometimes you just wait. I had a northern lights auto stop flowering after a week. Waited 3 additional weeks, flipped to 12/12 and upped slightly my bloom nutes, all it did was stretch, flowers stopped developing altogether. Only time it has happened to me. Genetics can be stable and 99 times out of 100 you grow a plant that is almost a copy of itself each time. That 100th time you get a seed that has some weird trait. Just a fluke more than likely. And besides, look on the bright side. You're gdt a better harvest of you let it veg longer. Always look on the bright side of life.