It isn't really. The people I know from Nottinghamshire would say they're from the Midlands/East Midlands and would get quite mardy if you told them they're Northern.This is all basically “I’m more northern than you”.
As far as northerners are concerned, the further north you go, everything below is the midlands.
By contrast people from Lancashire or above somewhere around Leeds probably won't know what mardy means unless they happen to like the Arctic Monkeys.
The key is, you can easily annoy people in Sheffield by telling them they're in the East Midlands, and it's important to take advantage of this at every possible opportunity.