There's also a load of other possibilities here.
Rider feels more comfortable turning left (or right), leans more or takes the turn faster maybe?
The route he takes has more turns in one direction than the other?
Highways (and most roads) are raked from center to the curb, so you'll have a slight bit more wear on the side of your tire closest to the centerline.
There's some other things, like the one armed rider, but we don't need to get into the crazies.