Like the person above me said a girl might be worried about losing her friend and she doesn't even bother to think if said friend is into girls without having asked. At some point I just can't even suspend my disbelief.
That's just like a general wishful thinking thing, but in that particular series there's an understood sense of mutual feeling, the uncertainty is with the degree, like almost always there's "is it the same kind of like?" that is in a lot of these. So while it's not overtly stated it is a cause for worry.
i'm not seeing much of this "gay is the default" in many stories outside of like ensemble type series? Which is fair because its under the yuri genre and that's the fantasy they want to sell... soooo..... ??? There's also the fact that like, queer people often form social groups in high school without them even realizing. Side couples tend to either play into this or else serve as foil to the main pairing. Most recent incarnations of "but we're both girls" have morphed into just more common/authentic queer awakening narratives instead of like, "Oh it's so taboo, it's so wrong!11!!11", the former of which I don't really take issue with.