I agree, I've dynoed Alot of different cars at the dyno at my school and 75% of the vehicles (stock from the factory ones) will have at least 3rwhp give or take from each run back to back. My teacher said he always saw the best numbers on the second run cuz the first run gets the cylinder temps up to temp, but I believe it's more than that also. We used an inertia dyno so all it does is calculate it from how long it takes to spin the dyno rollers which are a certain weight and mass (basically a big truck rear end underground as my teacher said). so if you mash the gas at a different time or have momentum while cruising up to your start speed/rpm(when it starts calculating how long it takes to gets the rollers going) it's going to be a different reading then if you slowly climbed up to that slower start speed/rpm and then mashed it. When I dynoed mine I kept it in mind and did the exact same everything and I would have .5 rwhp+- difference back to back. Coincidence? maybe, but my teacher said he never sees a car with such consistant results everytime.
Also before we did any runs in the morning we would run it till the oil in the differential underground would reach 106 degrees i believe it was. Who knows if that really mattered but that's we did. There's so many variables.