Momma cat adopts stray kitten?

My cat had a litter of kittens not that long ago ago. The other day though we found a random stray kitten in our garage. It is older than the other kittens so we know it is not our cats kitten and we had also counted them when they were born. At first my cat hissed at it but after a couple of hours my cat was letting the kitten drink her milk! I was wondering if this has happened many times before or my cat is just really special and sweet. :)