assisted_exposure means one character is exposed by a second character. But skirt_flip does not necessitate two or more characters. For example, the posts under skirt_flip solo seem to have the single character themselves flipping the skirt, or the wind/gravity flipping the skirt. These do not qualify as "assisted" exposure.
Furthermore, there are some skirt_flip posts, such as post #1324694 or post #733883, where the contents of the skirt aren't exposed to either the viewer or another character. These do not count as assisted "exposure".