When a powered rail is already powered, and you try to give it another stronger redstone signal, it will not update correctly.
Steps to reproduce:
1) Create a long line of powered rails.
2) Place a redstone torch at either end, this will power 9 rails.
3) Go to the end of the powered rails, so there is a powered rail next to another powered rail then an unpowered rail.
4) Give that rail power from a torch, repeater, button, lever, or pressure plate, the line of powered rails will not increase, and remains at 9.
Giving power to any location on a powered rail will extend how many rails are powered respectively.