Some artist constantly seem to blow one part of the anatomy. No one that skinny would have clearly visible ass cheeks hanging down in such a way as to be visible from the front through their thigh gap, matter of fact it'd rarely be that noticeable from the portrayed angle on any woman.
While the bad anatomy tag is still appropriate for this image due to the weird way her legs work, that's not "ass cheeks" showing from behind. Those are crurolabial folds, and they link the outer labia to the inner thigh.
