The cruise control has a speed limiter to keep you at the desired speed. E.G. Using cruise control while driving down a hill.
It is only Active Cruise Control that keeps the speed constant going down a hill. Standard CC only closes the throttle and if the hill is steep enough the car's speed can build up way above the set speed. Most cars since the '80s have had the latter.
I did a bit of an experiment in my MG5 Excite over the last couple of days.
There's a steep hill near my office.
At the top, I set the cruise control to the minimum speed, which is 20mph.
With 80% battery and KERS 3, the car held exactly 20mph all the way down the hill.
Went back the next day, straight from a full charge, and did the same thing.
With 100% battery and KERS 2 (because you can't set KERS 3 with a full battery), it did NOT hold the set speed but got up to about 30mph by the bottom.
I didn't try putting it in Neutral and freewheeling down the hill but I suspect I'd have hit a lot more than 30mph.
The inkling I got from this relatively unscientific experiment is that the cruise control WANTS to hold your set speed when going downhill, but does not apply the friction brakes in order to do so. Instead it appears to rely solely on regen - which it can't quite manage with a full battery.