@wasp; welcome to the forum. You are correct; the tyres on your car are not factory spec - at least not for a UK model.
The standard size tyres for the UK mk6 GTI with factory 18” wheels (7.5j x 18 ET51) are 225/40. Presumably the 235’s on your car are 235/40’s - this would give them a slightly higher sidewall than 225/40’s, which would probably give a slightly more compliant ride than 225/40’s. Maybe that’s why the previous owner fitted them?