I think we all know that in dynamic queue mid is the most wanted lane. This is also why we get this when we select mid lane as our prefered role.
I'm a top main, I can play anything but top really is what I'm good at, so I always choose top as primary role, and mid as secondary.
Logically I'd get top lane, but almost every time I get mid lane. This isn't the case when I choose for example top and bot lane, or top and jungle. In those cases I always get top.
The issue here is that mid lane normally is the lane most players select, thus it's logical players choosing mid as their prefered role have to wait longer. However, I always chose mid as my secondary role and I always get it instead of my prefered top lane.
It's quite logical that players who choose mid lane as their primary role will have to wait longer if everyone who chooses it as secundary gets mid..