[LeeCoUKFan]

Some of it is up to the ref.
It is hard, as a defender, to be in pursuit of a player and just stop on a dime because the ball-carrier steps out of bounds to avoid contact.Refs are aware of that.

But as far as thats concerned there is a difference between running into the offensive player and running into them or tackling them on purpose after they're out of bounds.

As I have always understood it, a late hit call is constituted when an offensive player is completely out of bounds and the defensive player either tackles or hits the offensive player hard enough to knock them down with the defensive player not making contact with the offensive player until the offensive player is completely out of bounds.