I would be very surprised if your cruise control kept within 3mph of the set speed at all times in all conditions. My car certainly doesn't, and it's a 2017 model with a sophisticated adaptive cruise control.

I'll ignore ACC for a moment and talk about the modern implementation of regular cruise control, however. You'll have your target speed stored in a control unit, which cares about 3 to 4 things: your set speed, your current speed, the flow of fuel to the engine, and in newer models, access to the brakes. So, how do we determine the current speed? The drive shaft will have a magnet mounted on it, and each time it spins around there is a sensing coil that sends a signal to the control unit. Based on the frequency of these signals, it determines what your speed is, and compares it against your set speed. If it is low, it will send a signal to a servomotor that is connected to the accelerator, sending more fuel into the engine, hopefully increasing your speed. If it is going to fast, it will cut fuel, and on some models, apply breaks if you are more significantly over the set speed.

However, this is a pretty dumb system. It only knows how fast you want to go, how fast you're going, and whatever the formula is that it follows for increasing the flow of fuel to the engine. It doesn't know about millions of other factors that affect how fuel efficient that acceleration is going to be. It is constantly readjusting based on the new speed readings. To my knowledge, all manufacturers also go for something that provides a "smooth" driving experience for their adjustments, rather than how to most quickly achieve the set speed. When you take the limited data available for a cruise control system and combine it with built in smoothing to keep the ride comfortable, you end up in a situation where the cruise control is making frequent adjustments that end up on one side or the other of your target speed. The lack of perfect information means they purposefully do not attempt to perfectly match the target speed - attempting to due so would require so many adjustments that it would be pretty terrible for fuel economy.

Now, hopefully, the Uber vehicle is going to have even more data available than my ACC equipped car. But even with my ACC equipped car, almost all of the work has gone in better determining what the set speed should be, and significantly less on making sure it hits it exactly. I expect this is probably the same on the Uber vehicle. Because quite frankly, it's just not that important. To be quite blunt, the difference between being hit at 35mph and 38mph is probably just how far your corpse flies.

As for saying they should drive 3mph slower than the speed limit, then they might end up going 6mph under the speed limit instead of 3mph under.

I suppose my fundamental argument in the previous post and now this is: What outcome is expected from them being able to stay perfectly at 35mph instead of having enough variance to hit 38mph? What benefits are there? A few mph makes the most difference in damage from an injury standpoint at very low speeds, and these are the speeds that variance is almost always significantly lower - there's no evidence here that Uber cars are going 8mph when they're meant to be going 5mph, for example, and this would not be the type of variance we see in cruise control systems in general - and very little difference to the outcome at high speeds, where the greatest variance is likely to be shown.