a typical 30/20 harvester collects 510/min gold at level 40.

if i put a matching crystal next to it, it still displays 510, but a green +20% appears on the icon.

if i give it a star (i.e. empower), it now displays 601, and a green +18% appears on the icon.
incidentally, 510*118% is about 601.

so my suspicion is that it does not harvest 601*118%, but that it only shows this increase redundantly. does anyone know details? did anyone measure it? if so i think it has to be considered a bug, because when i now add a crystal, it would display +38% but it would not produce 38% more than 601.