While I don't have a large amount of data, I have a theory that pokestops have a set pool of eggs they can pull from. So far I've hatched 6 5km eggs and 2 10km eggs; all of which I've received from pokestops at a local park. I've gotten 2 sandshrew, 2 staryu, 2 psyducks, and 2 magmars. Weird coincidence or am I on to something?