So what? Manga doesn't have a highly detailed photo-realistic style, rather it's highly stylized.
It's not exactly a secret that a lot of mangakas exaggerate sexual characteristics on characters in order to convey gender while avoiding breaking their unique style. Others opt for more arbitrary things, like reducing the drawing details on male characters, or opting for excluding eyelashes and drawing the iris as a black dot (while doing very detailed eyes on female characters). In many cases, if they didn't do that, the characters would all look the same (that's also why so many manga artists can cross-dress their characters so easily).
This author chose not to do that with this work, while making the girl a tall, flat chest tomboy, so you have to derive from the context that is given. These are one or two pages manga chapters too, so there isn't a lot of room for artistic and literary detail. Even so, this isn't an unrealistic scenario, as these tomboys do exist in real life.
I think too many male readers find this manga "confusing" because they are very accustomed and even numb to female characters with exaggerated sexual characteristics, and kinda "forget" they are looking at drawings in the first place.