I think it’s mostly out of habit, I have noticed that females who started nudism early in life are more comfortable showing their “privates” while those who got into the lifestyle later in life will still sit the same way clothed or nude.
It's 100% this! One of the best feelings about nudism is being able to relax about our inner parts. Those women you mention are not being modest, they do so unconsciously being taught from very little how to move to no expose themselves specially when wearing skirts or upper clothing that leaves the chest area exposed or loosely covered.