Well from my last grow, I had curved leaves at the top and also at the middle. I know for one thing applying pressure with sticks like I do may cause some leaves to curve a bit (LST with stick) .Also I notice some plants under strong light intensity tend to do this... well for me anyway... I would not worry to much as long the leaves are healthy and green. I have some girls growing naturally and away from the intensity and they don't curl. Hope that other growers may provide us with some of their explanations.
Cheers,
FarmBuck