I believe - don't quote me on this - that it's the person doing the kissing, whos score is taken into account. I may be wrong, and it may always use a combined parenting score of both parents, though. Do you know who the father was? Perhaps an unskilled brute initiated the kiss with the master parent, while a master parent initiated contact with your master.

As always, there's an element of random chance; unlikely things like that just happen sometimes..