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Some cats are obsessive about covering their waste in the litter box. Many barely cover at all, and some (like your kitty) over do the whole process. Believe it or not, this is 'ingrained' behavior in most cases, and a cat will follow what THEIR mother did in the litter box almost always - regardless if they were actually raised beyond litter box age by their mother or not. Sometimes, removing the waste from the litter box immediately will help stop the obsessive "dig to China" syndrome that some cats have, but often, it's a routine with them, and they'll just keep on digging regardless if your remove the waste or not.
Your kitty is normal - perhaps a little more enthusiastic than most about litter box habits, but don't worry about her. If she ever produced kittens, in all likelihood, some, if not all, of HER kittens would be the same way, believe it or not.
Please let me know if I can help further.