I don’t think it’s useful to obsess over the past. The only thing I would observe is that admissions officers are looking for children who (in their judgement) will do well away from home at that point in their lives. They are not looking for the “best” students, nor for the student who will be the “best” adults.
Your daughter (from your description) is very academically advanced. That doesn’t necessarily mean that the rest of the boarding school experience (or any high school) will be easy for her. There are think tanks which have recommended accelerating prodigiously gifted children, rather than attempting to keep them on grade level.
It is hard to be adult in one aspect (academic achievement), while still being an adolescent. Boarding school admissions officers also have to think about how the students will fit together, whether they will coalesce as a group.
On the bright side, an offer of admission does mean that the admissions people at that school believe she can do well there. It would be most helpful (as LaxPrep suggested) for you to stop looking over your shoulder.