Autos tend to grow very slowly until the flowering stretch 3-4 weeks from planting the seed. They dont need a whole lot of light during that time. When they start flowering, they grow explosively and you want as much light as you can give them. One of the things about autos is that you can flower them under 18/6 or 20/4 or even 24/0 which means more growth since they don't have to sit idle in the dark for 12 hours every day. Plants do need some dark during a day which is why people do 18/6 or 20/4 instead of 24/0