I think if the rotors are pretty far from the PRV that could cause pressure loss but I have never done the math to see how much of a run would cause significant loss.
I don’t think that the rotors are terribly far - one is virtually on top outside drain spigot
Although, while I do admit that I don’t know the route(s) of the trunk piping, it seems to me that if the loss was a result of the piping lengths, wouldn’t the rotor closest to the drain spigot have much higher pressure than those further away?
That’s not the case, as all of the rotors have approximately the same pressure.
Here’s another question - should the PRV be set so that I get the desired pressure at the rotor nozzles, as opposed to at the drain spigot?
I have seen some constant pressure vacuum breakers with up to a 20 pound spring in them. That means you will loose 20 pounds before you get to the manifold. Also the breaker needs to be one foot higher than your highest head which can increase the pressure loss before the manifold. Double check valves are usually around 5 pounds for each check spring (10 pounds total) but can be inline with the rest of the plumbing.
Were you running anything else while doing the first test (other zones, shower, washing machine, etc.)? Besides that the only other thing I can think of is that the water supply for your city has fluctuations in water pressure.