Its not peculiar to us.
Newcastle and Gateshead are in different historical counties, are in a new Metropolitan County (Tyne & Wear) but in distinct different Districts within.
Most of Newcastle would identify as Tyneside, but many passionately as Northumberland, never Durham.
Gateshead folk might say they're Tyneside but never Northumberland, definitely Durham.
When a major river divides counties, the settlements either side will not easily concede identity.
We have Tees Valley for Tyne & Wear.
Durham/Yorkshire for Northumberland/Durham
Newcastle is the most significant settlement on the Tyne and is the City.
Middlesbrough should attain the same re the Tees.
Liverpool, Newcastle, Glasgow, Dundee, Hull, Sunderland are the Cities,
not Merseyside. Tyneside, Tayside, Humberside, Wearside etc.