The truth, Marianne, is that you are onto a key fallasy within the world of publishing. (Mind you, it doens't matter how much you educate people about it, they resort to their use of the mashup you have called out.)
There are, in reality, three types of genre. The problem is that segments of these different genre axes are merged into the general classification.
The first is audience genres. These are such things as age-, gender-, race-, or culture-defined genres. (As though audiences are so shallow.) Women's fiction, as you mentioned, is one of these. YA is another. There is a little more logic in the younger age-related ones, such as MG and childrens, as those pertain to the complexity of the stories.
The second is presentational genres. These are when the genre is defined by the setting. The most common are the likes of sci-fi and fantasy, but western and military also fall under this umbrella, as do certain culture-centric genres, especially where the culture is presented through (or for) its otherness.
The third is story genres. These pertain to the structure of the main story elements, and include the likes of thrillers, relationship (including romance), mystery, horror. (There was a whole year's worth of Writing Excuses podcasts on this type of genre classifications.)
Obviously, every story is going to have elements from each of the three trypes of genre definition. The challenge comes in deciding - as with the example L. cited - of which is considered dominant. Is it a romance that happens to be sett in space or a near-sci-fi that happens to include a love story? Or is the fact it's aimed at a younger audience more important to the marketing.
And that is largely what it comes down to: marketing, The mashup seems to be a range of (time-changeable) genre categorisations that many reader choose to identify with. They decide they like X but not Y, so how the book that is both X and Y is marketed determines whether they will even pick it up… And what they will think of it once they do.