Also the whole house cup system and houses were not a big deal.

They were a big deal to Harry when he first came to hogwarts; because that was his first sense of community, and friendship. Most of the slytherine are just like “oh that sucks, anyway: yeah can’t wait to go home.”

Even for Harry the whole house cup and point systems fades as he realizes it’s just a silly way to help keep kids in line. About halfway thru the series they don’t even mention the house cup because Harry doesn’t even think about it anymore.

So giving grfandor the house cup was clearly a “aww it means a lot to him” thing.