I always saw it as Nanoha just devoting like everything to defense and waiting til Fate to waste all her energy. Or Fate not willing to go all the way like Nanoha wants to. According to wiki, Starlight Breaker reuses left over energy from magic used on the battlefield which makes sense but that's sort of an important plot point that was left out then.
And yea, it'd be nice if Nanoha had suffered more damage.
edit: Oh, I am really stretching to defend this now.
