My neutered Maine Coon (who was neutered at 6 months of age) occasionally tries mounting one of my spayed girls and I was concerned about it until someone told me that it's a dominance thing, it's him showing her *who's boss*
The only thing is he's so sweet and docile that I found that hard to believe. He's just so easy going and lets the girls push him around.
I had also heard that if the vet only finds one of their testicles when they are neutered, the other one could be inside of him, and therefore the cat would still be prepared to mate. But I did ask the vet if they removed 2 of them, which they said they did. I would think that if the vet only found one, they would have said but I thought I better make sure.
I have a 20 week old British Shorthair male who is due for neutering soon, so we'll see if he does the same thing!