Its not too complicated. I am saying some of these prep schools have their grades LOOKED at differently than a public school. It is entirely possible, and I have seen it happen to multiple friends, family friends, and siblings.
A Phillips Andover/Exeter/Middlesex/St. Pauls etc. all have their grades looked at differently because of the difficulty of their classes. A "B" at one of those schools is the looked at as the equivalent of an "A" at most others.