This is because it is the financial institution that filed the return, rather than the individual, and it is the financial institution that paid the devolved tax and so may make a claim for repayment.
Mae hyn am mai'r sefydliad ariannol a ffeiliodd y ffurflen dreth, yn hytrach na'r unigolyn, ac mai’r sefydliad ariannol a dalodd y dreth ddatganoledig a gall felly wneud cais am ad-daliad.