It is historic in many ways and also exaggerated through the influence of Victorian Travel writers like George Borrow and his very well read "Wild Wales".
There is an imaginary line called the Landsker line and the story goes that those below were mainly Non-welsh speakers, (not necessarily English speakers, but more recently certainly) often claimed to be of Flemish decent (not sure there's much evidence, but hey ho). The line has moved up and down over time as Welsh speaking areas grew or receded.
I have always known it as Little England beyond Wales and it's just a bit of fun really - I think people from that area were always considered Welsh, but very English in their ways and their language. An English person who has moved there and maybe thinking its part of England, probably doesn't quite get the history of it.
P. S. I did say to someone from somewhere South of the Line once, that he was from Little England beyond Wales and he was very offended, because he regarded himself as very Welsh and I had to very quickly take it back.