Wind direction and RPM
Most of you that are flying fixed pitch aircraft will have a "cruise " prop fitted , these props have the pitch ( or A of A) set to give the best performance in the cruise airspeed range go above that cruse TAS and the A of A of the blade decreases and therefore the "lift" that the prop blade gives falls so the aircraft wont go any faster.
At the other end of the airspeed range IE at rest the prop blade is fully stalled and is no more that a wind shovel !, this is a very high drag situation so the engine RPM will not be as high as when the prop is working at its best TAS.
Doing the engine run up into wind will get result accordind to the wind speed after all a 15kt headwind is more than 10% 0f the speed range of most fixed pitch light aircraft and will more than likely unstall the prop giving an over optimistic static RPM , with a tail wind the oposite would happen.
It will come as no surprize to to you all that when doing a C of A airtest the static RPM check is done crosswind so that the wind speed will not effect the static RPM result.