Girls, Why an underwear model were so worried about their underwear (or event pants) once they wearing skirt as model?

Hi there. When I watch model photograph session video, Sometimes I find female underwear model, casually taken a picture. She's look so confident. But somehow when this same person become car / boat models, or any other models, when they wear miniskirt, they are so worried with their skirt, they keep holding it or cover it with hands. Worried so much that their underwear, or even short pants, exposed. I wonder why. They are underwear model anyway, and get used to it. And the pants are covering more anyway, since they aren't wearing just underwear like when they modelling for underwear. What they affraid to be seen? Or there's another reason?

Girls, Why an underwear model were so worried about their underwear (or event pants) once they wearing skirt as model?
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