I actually think it makes perfect sense.
Mid is actually the most popular. Mid players are subject to wait the longest for their role. However they are probably also the most likely to get auto-filled. I was never autofilled when I queued top/adc, but since switching to top/mid I've been auto-filled somewhat regularly. Since the role is more valued it's harder to find a spot for it and easiest to just throw them on support. After a while, the queue gives up on primary over secondary, and this happens long before it resorts to auto-filling. So the guy with mid secondary is just as likely to get the autofill as mid primary, and the other gets mid.
Also of note: I queue top/mid and get top like 80% of the time. So I don't think it's really that way. Maybe your elo specifically, but the message is constant across all levels? Which would be a shame, the message should be dynamic.