Sounds like the cash figures you have been quoted are correct.

However can I check what you were trying to do?

If you are doing book J GUF to F as a return you would pay the J avios for MIA-LHR + J avios for LHR-SFO.

When doing as two one ways were you looking at F avios for MIA-LHR + J avios for LHR-SFO & GUF to go to F? That would be more expensive in terms of avios than the return.