Yep, what Opjose said.

to expand a little, all engines will get some kind of RPM change when you change the pitch or diameter or even brand of prop. Engines are non-linear, and the aerodynamics of props are fairly complicated (though you can simplify to "rules of thumb"). So different engines will react differently to prop changes and just changing a prop can really change the performance of an engine and airframe.

Fwiw, there's a guy in my club with 2 Alpha 60's, both put together from the ARF. One has a K&B Sportster .61 and the other has a Super Tiger .90 in it. He used the Alpha's 3-bladed prop on both engines, and they pretty much acted like the Evolution .61's act in the air. (well, except that the ST .90 could over-rev and had much more power).