She wasn't doing 5th grade work so you'd expect her to lose some of the specifics of 4/5/6th grade work after a year especially if it was no longer relevant to what she was learning.
There is probably some scaffolding work that they do in the 5th grade that becomes meaningless once a student advances into higher grades and no longer needs the scaffolding.
Or it could be a question around some trivial side lesson that was done "for fun" but that noone cares about or remembers after 5th grade.
Or it could be she had a little bit of luck in the last exam (guessed the correct answer on limited knowledge) compared to this.
She is so close to the ceiling that even really minor things going wrong can make results look bad. If she is doing well where she is then I would just right it off as one of the quirks of life.
