Rams Horn, claw, leaf curl

Are their some strains that will claw no matter what? I always thought it was a sign of nute burn, but I've seen it while running low ppms as well as high. I also get the idea that leaf curl means different things for dirt plants than it does for dwc plants.

any comments?

Here's a few more observations...
I get more claw, or upside down canoe shaped leaves lower in the canopy, and further from the lights. Close to the HPS, no claw. This even in a 75-80 degree room. Some plants claw alot, some not at all. Claw seems to intensify during early flowering. PPM, high or low, plants will still claw. After spraying my leaves with a little mist of RO water I had more clawing, not less. I run about 35% humidity when the lights are on.

My hypothesis is that the leaves are in fact cold. Did not believe that could be a problem at 75 degrees, but I'm starting to now. I believe that my low humidity and high leaf transpiration are causing evaporative cooling. When I sprayed them a little, they curled up even more within minutes.
I'm a RDWC grower, so I really don't want to raise my temps. I guess I'll have to get the humidity up a little instead. I might just ignore it, the plants still make nice buds even with a little curl. My Sativa, the Maui, has no curl at all and loves the current humidity.

guess it comes down to genetics, interesting read, i have got some curling rite now as well, but i figured it was due to all the crystals, and the fact that im getting close to harvest.

it might be stem or root rot...

I have some "claws" on a few of my plants too...I don't think I'm over ferting as my plants are almost a light green and my ec is 1.1(550ppm .5 scale). The temp around my plants is 22-24c(72-75). I was flooding every 6hr for 10mins, but yesterday I turned it back to every 7hrs. I adjust pH to 5.5 before going to sleep, but will find it around 6.0-6.2 when I wake up.

Any ideas? Can this be a sign of trouble in the roots?