It does seem odd. The relationship of Guy and Sheriff is unclear in folklore, as well. There is a medieval ballad in which guy is represented as a sort of murderous vagabond, but that's likely to be satyrical. For the Sheriff of Nottingham to have been given that office, he must already have been of baronial rank. Prince John may have been paranoid and nepostistic, but he is unlikely to have ignored the shrewd convention that a shire's "Mr Big" should be charged with upholding law and order therein. So, if Guy of Gisborne was the resident abusive aristocrat in Notts/Yorks in c.1200, where does he end and "The Sheriff of Nottingham" begin? Could they be one and the same?
I quite like the movie-convention of keeping both seperate, however. Gisborne works well as the sadistic toff allied to a sort of ruthless civil servant in the shape of The Sheriff. I would possibly go further in diversifying the baddies by borrowing Front de Beouf from Ivanhoe.