"And in Sweden everything is a polska (which is NOT a polka)."

Well, not everything. Maybe Richard Mellish should come in here.

Whichever tune is chosen, what matters is that the rhythm of the tune matches the steps the dancers will be doing. Though mistakes here can have interesting results. I was once at a dance where the caller called The Rifleman, walked the dancers through it and told them to rant. The band then played a hornpipe. For those of us who ignored the instructions and did a hornpipe step it was BRILLIANT.