@Linkat,
Sometimes, actually a lot of times, they do it on purpose. I have family from the East Riding of Yorkshire, some of them deliberately talk in dialect because they think it's funny when you don't understand.
When I was little at the dinner table my grandfather asked me if I wanted some air. I didn't know what he meant, but that didn't stop my father telling me off for being rude. My grandfather then said, 'A bit mair (more) ham.' which he thought was bloody hilarious, but I still fail to see the joke.
Having said that in the Alan Partridge sketch the actor playing Geordie Michael originally spoke a lot slower but the director told him to speed it up to add to Alan's confusion for comic effect.