seanychen,
I cannot explain the difference in readings between my results and yours, but i will offer these thoughts.

True, Atmospheric conditions and altitude will make significant impact on an engine's ability to perform. Fuel will also, but in my experience, not as much.

I have also experienced 100-200 rpm differences in the same size and manufacturer's prop. For instance, I have tried two Zinger 15x6 props on my Saito 100 and gotten a 200 RPM difference in peak. When my current Pro Zinger 18x6 wears out or breaks , I may get slightly different results with the replacement.

In your readings I would investigate why the Pro Zinger 18x8 turned 600 more rpm than the Top Flite 18x6. They have a very similar shape and profile. An 8 pitch should offer significant more load than a 6 pitch. Something just ain't right there

The only real way to make meaningful comparisons is to put the Engines side by side in a stable atmospheric environment; and use the same fuel, plug, muffler, props, and tach on each.