Er, I don't know
In my recent babbling about which collective noun best describes residents of Springfield, I explored which of the word endings "-ian" or "-er" was most appropriate to attach to the word Springfield.
The Brits seem to have settled on "-ian," but opt for making it part of a wholly new word. Thus Oxonian, Glaswegian and Mancunian for a citizen of, respectively, Oxford, Glasgow and Manchester.
This is not mere British eccentricity. Oxonian derives from the Latin term for Oxford, and has been in use since the 1500s. Ditto, in general terms, for Mancunian.
The origins of Glaswegian are murkier. It might have been something that Sir Walter Scott made up, which would be just like him.