They would all be equally successful - basically it doesn't matter how many results a combination might have. First you have a chance of getting a common, medium or rare dragon, then you have a chance of getting one of those dragons in the sub-category. I think rare dragons at something like a 1 or 2% chance? There are wikis or pages on this board that would explain it in more detail.
So when picking a combination, the best thing to do is to try pick a combination where the failure dragons either do not take a long time, or where the failure dragons might be dragons you still need for your collection.
It's possible those were the pairs with the shortest times when you looked it up and the pairs that are shorter now produced bone or ghost or some limited that isn't around now. Not sure. --- "In your face Lord!" - Homer Simpson