Interesting observation, PT that the cramps never effect the arms. I also never get cramps on long walks, but I do on long swims. I get them normally, so often that my cool down is usually breast. That helps to flex my legs and release the cramping.
We spend most of our time with our feet in an "L" position, pushing off from a solid surface with a backward thrust. then we get in the water and change the whole pattern. We point our toes, move our thighs back and forth while relaxing from the knee down. Our legs get confused.
Our arms and hands do all sorts of differently shaped motions all the time so they adapt better. they arms get crampy from writing, typing and other unnatural motions, not swimming.
Just some thoughts on the subject.