>>713499243they're simply not very well drawn
look how the line from her armpit doesn't really match with the shape of her boob under the dress or her other boob
it seems like her back is actually part of the shape of her boob, it probably was like that in the original sketch and when the artist realized her boob was going under her arm he tried to fix it like that instead of redrawing the shape
also giving her a more voluptuous body but still keep her twig arms looks weird too