Flowering during growing season? Experienced grower Q.

I would like to know why or if anyone else has had plants show there sex after only 5-7 weeks old under 18-6 light. I have heard of Auto-flower so the first few grows i thought maybe they were that, but i have had several different grows and lots of diff strains w/ at least one if not multiple plants show sex around 6-7 weeks old. It usually is correct except a couple times it ended up to be a male anyway so i dont trust it 100%.

Any insite would be greatly appreciated.

Fox Farm Trio 400W HPS metal halide conv.
24-0 for the first 2 weeks.

yes i have had this plenty of times ... some strains will show early sign and some wont... your lucky if they do man