I've noticed that the higher my level gets, the more rare/super rares I seem to get. I got a Metal dragon, a Disco dragon, and a Social dragon just in the past two days. Also got two Cloud dragons, another Landworm and another Atlantis last week. All without boosted breeding! Sure, I'm still getting commons, but more like 2/3 rather than 9/10.
I heard a rumor that your social score has to do with it too, but I've been maxed out for ages, so I don't think I buy that.