Write it down to the sadly typical human insistence on feeling group superiority over someone. Plus far as I understand it the southern Greeks, not entirely unjustifiably, basically regarded their northern Macedonian cousins as a bunch of rustic country bumpkins. The usual "returning the favour" in such circumstances tends to involve regarding your more urbane neighbours as soft and spineless, nevermind now if you just beat them up in a war...
And of course the Macedonians in turn then were all snobbish regarding all those "barbarians" north of them...

Ecce homo. Everyone likes being able to look down on someone.