You may not have floaty legs at all. There are a few minor things that you could be doing which are causing you center of gravity to shift forward slightly resulting your legs up position and the illusion of floaty feet. Without diving with you or having any pictures or video or in fact any idea of how you're diving it is a little hard to say what might be going but I'll give you a list of a few things you can experiment with the next time you're out.
On the technique side of things:
- Arching your back. Arching your back causes your center of gravity to move forward. Maybe you are arching it quite a bit and not noticing.
-Positioning of your legs. The position of your legs can make quite a difference. If you are going for horizontal trim and trying to look like the guys in the wkpp videos (flat with knees bent around 90º) it is possible that you are bringing your heels a bit too close to your tank (moving center of gravity forward), try extending your legs out from the knees to shift your center of gravity back. This technique works best if you are using a frog kick rather than a flutter. Dropping your knees will shift your center of gravity down your body towards your hips, but it is bad form.
On the equipment side of things:
-Fins. Do your fins float? Floaty fins can be a pain in the butt. Ankles weights can be a good way to compensate, but I'd try to learn without them, that way if you ever end up without them it doesn't really matter.
-Tank position as mentioned earlier.
-Distribution of weight/BCD fit. You mentioned that your weight is evenly distributed in your bcd. Great. Now how does your BCD ride on your body while your diving? Does it move up towards your head? It is possible that evenly distributing the weight along your bcd isn't what works best for your body composition, you might benefit from moving some weight onto a weightbelt.
As for needing more weight above 35ft, that is a physics issue and has absolutely nothing to do with your chromosomes. The deeper you dive the higher the ambient pressure and the more your wetsuit will compress. The more your wetsuit compresses the less it floats, requiring less weight to compensate for its buoyancy, so if you are conducting a dive around 100ft chances are you can get away with using less weight and just swimming against the buoyant force of your wetsuit in the shallower water. Not really the best idea though, you might want to check your weighting the next time you go out.