I suspect this goes by sex, or USED to. Men wearing pants always slam their legs together to catch something dropped.
Ladies, those who always wear dresses automatically spread their legs to catch things.
Seems to me that one of the old Mark Twain books had some guy catching a boy out who was dressed as a girl. When someone dropped something in "her" dress, he slammed his legs together to catch it, while a girl would not.
Personally, I rarely wear a dress or skirt, so always put my legs together to catch anything dropped. Sometimes this is not the best thing to do. This is a learned reaction so not much to do but be very, very careful when using sharp objects.