More likely reasons for the difference would be trapped air in the BC
I agree with Walter. It's more likely that there was air in the BCD, and that when you hit 40 feet, the air in the BCD was mostly compressed, and made it easier to remain neutral. The same air (that probably didn't properly vent) at 14 feet would be more buoyant.
Another possibility is the compression of the wetsuit, and that at a shallower depth, the wetsuit is less compressed, and therefore more buoyant, but at depth, it's more compressed, and less buoyant, and easier to maintain neutral buoyancy, when the diver is underweighted.