Well, ok, thanks. Erm... it eliminates my misunderstanding the rubric calculation. It must be something else.
I did check for the usual suspects like locked grades but the gradebook seems "clean". It's not a "sample of one", all the grades seem (approximately anyway) like they are divided by four.
EDIT:
Just went back to the grading page and re-saved the page. The grade is now correct. Don't know what happened (I'm just the support guy so don't know the 'history'). Looks like we have a solution