It's possible that's what happened. I think I usually took the cover off first then turned it on. Later I uncovered and turned on and then made final height adjustments to give a bit of time for the sensor to stabilize. Finally on the last few tests using the aquarium hood, I just left in on and uncovered because it didn't take that long to get the height adjustment right.
Good to know though. In the future I'll make sure to turn on, then uncover.
If I get time I may test those spiral CFL's again. Just to see if I can get more consistent readings.