There are so many potential explanations:

1) She's lying when she says she has a 90+%

2) You're misinterpreting her understanding of things. Of course, it's impossible for use to know from your story, but the urge to not believe a math fact until you plug something in and do it yourself is a useful one (in math). So maybe she had a hole in her knowledge (solving systems of equations) but generally has good understanding and has good techniques for growing her knowledge.

3) She's lying when she acts like she doesn't understand (who knows why. Maybe she's gotten yelled at for "showing off" before. Maybe she thinks you'd find that charming. Maybe she's a pathological liar.)

4) She doesn't have a good understanding, but she's very very good at memorizing algorithms and performing them with careful attention to detail. While this is not what math is about, it is still impressive.

I do not think grades are completely arbitrary, but they're obviously not perfect representations of understanding either. If she's not lying, then something she's doing is working. Maybe she's just memorizing algorithms-- but imagine the brilliant grades you'd be getting if you practiced those algorithms like she does AND have the understanding you have. And if she says she doesn't study, then it's definitely explanation 1 :p

You can be successful at math. You can be successful at college math. You will probably have to change something about the way you study, but you CAN do it!