My guess in this case is that you can strip off most of that technical speak and just say that the system may be throttled to about that speed, for whatever reason perhaps the car's drive system slightly misses 65. I'll wager that the vehicles should hit about the same speed whether they're loaded or empty, the speed is probably tightly controlled. The electric motors are probably capable of going faster, although it would probably heat them up more.
I really thought I got a good GPS track of the outside portion of that ride, it should have allowed me to get a fairly accurate speed for the second or two before you pass that speed sign. But.. I've looked and looked, and all I can find is a partial track that doesn't get above 50 and seems to be the portion when you're heading away from the main building, not the speed run coming back.
Perhaps they intentionally set the speed to be slightly under 65 to add to the suspense, to give you the feeling that they're pushing the car to try to get the last bit of speed out of it, the feeling of having that little bit more to go just before you hit the banked turn.