Let’s learn a new word: superfecundation. Based on Latin, it means “overfertilization.” In plain English, a female cat in heat can become pregnant by more than one tomcat at one time. That is, a litter of six kittens might have the same father—or different fathers.
Tomcats are not even remotely monogamous, but neither are females, and a female cat in heat has no qualms about being mounted by more than one male, even on the same day. If you understand superfecundation, you will have a better understanding of how kittens in the same litter can look so different.