Milestones are not called mile suggestions. When you are on a jogging trail and you pass by a mile marker, it doesn't say "somewhere between 3/4 of a mile and 1 mile." The mile marker is very specifically at the 1 mile mark. Same thing with milestones. They are the way the infant and toddlers' brain literally turns on or boots up. Sure there are variations, especially with preemies, but for the most part, certain things should happen within very certain times or else concern should be raised.