I would guess GLA provides a more attraction choice of routes to those in the north of England than what EDI does.
And you host a webpage to prove it? Remind us again the URL?
Let's check the surface mileages.
Newcastle to GLA = 157 miles
Newcastle to EDI = 110 miles
fwiw I doubt either EDI or GLA will benefit from any NCL imbalance of APD levies.