Recall that the magic a girl gets isn't always closely related to the content of the wish. I don't think the incubators have a lot of control over it, so my guess is that you get magic according to unknown traits unless you make a wish for a specific kind of magic. Wishing for a specific kind of magic seems often to result in broken effects, and not in the good sense, such as Grief manipulators only being able to move it from gem to gem. So, my hypothesis, which is untestable at present but could show up in the story some day, is that people have natural magical affinities. If your wish isn't for a particular kind of magic, the wish can be granted without using up the contractee's 'potential', and they have a strong affinity for a certain kind of magic, then they get the wish & an unrelated primary magic, such as Kyoko wishing for people to listen to her father and gaining the magic to duplicate herself.
When you wish for a specific magic AND you have an affinity for it, that's when you get the super strong outliers like Sabrina.