There is an extra "peg" in the front (or was it rear) hub that prevents you from putting the wrong wheels on. This is because there is a matching hole on the front wheels (or was it rear) but not on the rears. However, these pegs can be removed easily once you get the wheels off.

I had to do this the first time to get my winter wheels on, and i have never put them back since then.

Hope that helps.