The wonderful thing about having two separate variables is that they don't have to both increase at the same time. It is possible for the max to even be below the actual. If, for instance, a unit has actual=2 and max=1, then the unit can move 2 tiles on each turn, but both tiles must only have a requirement of 1 movement point each. This will allow great flexability (as I've said) in designing and promoting units. Do you want to give more general movement, or give the unit ability to enter into more types of tiles? The choice may be yours.
As regards the enter tile chance, if that is implemented, it may always apply in some measure I think, even if the actual amd max values are the same. For instance, if a unit has actual movement of 2, and max of 2 (allowed to enter forests), the unit may first move into a 1 movement square. If it then wants to enter a forest, it can only attempt to that turn, the chance of success being the actual movement remaining divided by the movement required by the forest. If it fails that turn, the next turn the unit can move in normally, using all its movement points in doing so.