Originally Posted by
[S8] Elsa
Boosted Breeding increases your chance to breed Rares, but it doesn't guarantee it. So while you have a higher chance to get those Dragons, you still have the opportunity to receive Commons.
With that being said, each breed is a new roll. So, you could breed 15 times normal and get Islands for all of them or you could get lucky and get the rare you wanted the first time. What Boosted Breeding does is slightly boosts your chances of rolling a Rare for each breed you use it on.