Engine Vacuum vs RPM?

If it was under a low load, then the carb would be mostly closed and the vacuum could go up. So your hypothesis on what they were doing may be right.

The reference to vacuum dropping under load is that it drops more and more with increasing load. With a fixed load, then vacuum may go up with RPM. (Even at WOT it ought to go up with RPM, even though it will be near zero.)