It's likely software limited at high SOC% to prolong battery life. Much like how DCFC tapers above 80%.
I wonder how a Gen 1 Volt could do 60kW regen when MGA (generator motor) is rated at 55kW. In the many years I used my Gen 1 (2013) Volt, I have never seen regen go over 55kW, even when checked via OBDII
MGB is rated at 110kW and is always connected to the wheels.
Even then, motors aren't too particular about exceeding their rated values for short periods (within reason, not like 10x).
I have seen it hit 60 occasional. By far the norm is 55 max.
IIRC all the times I've seen 60 it has been in a combined mode (both motors connected in the transmission, with or without engine on) going down a hill at a decent speed and coming to a stop before it could clutch out the second motor.