Here's just an observation from my own personal experience. I've taken several cats and have had ears "tipped". But I also had this tiny kitten show up with the top part of his ear injured and it soon fell off. So now my dear Higgins has a "tipped ear", but he wasn't fixed until I had the vet do it. So make sure that the ear that is "clipped" isn't a result of an old injury instead of being sterilized. How you do that is another question; one for which I don't have the answer.

ETA:
I've attached a photo of Higgins so you can see the ear.