Faire partir une auto-floraison en 12/12

according to my humble opinion and vision, it seems like mine with the other days, a difference instead of my internal growth with the lamp, so a little bigger or less does not make a difference, the little difference you noticed it can be given by a lot of variables such as temperature, light absorbed by nutrients and oxygen etc. etc. so I advise you not to worry and not to be in a hurry and not to stress too much, it has its time and more time you can wait without interfering too much better and, from what little I learned to learn and that if you want a good product you need with a lot of patience, try to check the pH of the soil which is perhaps one of the most important things, but perhaps this thing you can correct slowly and always a little at a time. I wish you well for a good production.

I would keep it at 21 hours if I were you. The whole growing and flowering process should have 18 hours of light. Autos do need some rest to get strong and be able to hold those heavy buds later on.