How can kittens from the same litter have different fathers?
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This occurs not just in cats. It occurs because two or more eggs are exposed to two or more sources of sperm. Cats frequently ovulate multiple eggs–the kittens in a litter are usually fraternal twins (multiple fertilized eggs by different sperm–they are just siblings who happen to be born at the same time) and not monozygotic twins (identical twins which are genetic clones of each other). Cat’s aren’t monogamous and so they may “collect” sperm samples from multiple partners which can fertilize different eggs.
Human females normally ovulate a single egg per month, but it is not super rare for two eggs to be ovulated. This is the source of fraternal twins. If a human female has only a single sexual partner, then the fraternal twins will both be fertilized by the same father. If the human female has multiple sexual partners in short period of time then it is possible that each egg is fertilized by a different sperm resulting in heteropaternal twins (different fathers). According to a Guardian report about 1 in 400 fraternal twins have different fathers.
In human populations the fraternal twins happen about once in 85 births; however, there is a genetic component and some populations are more likely to have fraternal twins than others (africans are more likely to produce fraternal twins, and asians less likely). Due to the genetic component, it is also possible for production of fraternal twins to run in families.
Monozygotic twins occur about once in 250 births. The reason why the fertilized embryo splits is not understood. The incidence of identical twins does not appear to have a genetic component–the odds are about 1 in 250 regardless of the population of the parents being monozygotic twins.
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