There is no one answer, but hopefully this will help.
First things first. It's not true that HS girls as a whole tend to perform better in races than workouts. They're no different than guys in that regard. In my opinion, it's also not correct to think that she is training as she should, but given that she's apparently running alone (i.e. friends aren't holding her back, which is pretty common), it sounds more like she is holding back in a big way. The real question is why?
Several possibilities.
1. She has a parent or "inner voice" telling her not to overextend herself in practice and that only the big races mean anything.
2. She's lazy and/or unmotivated to do the work.
3. She just doesn't get the whole training makes me better thing, at least not in a way that impacts her choices at practice.
The best solution would be to talk to her about your perceptions, quite obviously if you haven't done so already. I wouldn't confront her over it, but ask if there's a reason. What you want is a self-motivated kid who will choose to work hard, not a kid who feels forced, so I would be sure to tell her that she's free to make whatever choice she wants, but that she could achieve so much more by taking a different path. Tell her that you care about her as a person, not as some coach who wants to squeeze everything from her that you can, and you just want to know her thoughts so that you can make her experience as positive as possible.
Again, not to say you aren't doing it already, but I'd follow up by making sure you have a good balance with any praise toward the team, by talking as much about effort as performance, if not more. E.g. rather than talk about how great someone's time was in a race, or how they won or finished high in a meet, I'd talk about the effort kids put into practice and how it paid off in the race, or focus on effort during the race. Maybe a kid got out slower, but came on strong, or started too fast, but hung on. If this kid is into being praised for her performance, then maybe switch the variable that governs praise to something like effort.