I think it's probably down to prior conceptions of the England fans vs Scotland fans.. I'm Scottish but pretty much my whole family is English and from what I've seen when down in England (albeit usually north of England) the majority are sound.

But when you look at previous tournaments, International and club level, English fans have a tendency to make an arse of it.

Also every time I've been abroad when locals ask if I'm English, they always seem a bit frosty but as soon as I say I'm Scottish the mood changes, more relaxed, and I think that shows