It seems to go in spurts for me. For the week after the Egg event I would say 80%+ of my eggs were 2km. I couldn't get anything else. My bag always had 7+ 2kms. Then I went on a run of 10kms and got 3-4 in quick succession.
This past week though has been all 5kms. I don't know but they seem to come in bunches - whatever the eggs may be.