Ovulation
Female felines do not release eggs from their ovaries until stimulated by the mating process. This is called induced ovulation and usually occurs from 20 to 50 hours after mating. Miss Kitty is likely to breed several times, and with different tom cats, before her body tells her ovulation has begun. This is why her kittens can have different dads -- there can be sperm from several different males in her reproductive tract as the eggs are fertilized.
Superfecundation
The term for two or more eggs from the same heat cycle being fertilized by different sets of sperm is called superfecundation. It is not unique to cats, but is also common in dogs. Although not likely, it is theoretically possible for every single member of a litter to have a different father. Though extremely rare, it is even possible in human twins!