In my opinion, because with FTTC there will be people living on top of every cab (at the end of their drive, against their wall, etc).
This means that one of these people could end up with, potentially, a 120mbit sync. This would cause lots of crosstalk to the lines that were longer.
By limiting such a customer to 80mbit, you can reduce the bitloading and power levels, meaning less crosstalk to longer lines.
It's a bit of robin hood going on, taking from the fastest to give to the slowest.