Probably it's more that non-type A people group with other non-type A people. So your group of non-type A mountain bikers fail to see the type-A groups of mountain bikers because there is a "colored lens" thing going on. However the glasses come off and your bias to road bikers comes out by manifesting itself with your subjective observation that road bikers are type-A.

I'm sure there are a decent representation of both personality types in mountain bikers and road bikers.