it is longitude gain problem
don't know exactly what is meant by that....
Any change in rpm will change the pitch, it's just a matter of degree. 1% is not a lot and normaly should provide an acceptable thread, but...not sure what tolerance you are trying to achieve. If mach sees a change in rpm, after the first thread pass, planning is done to provide a correction to the next pass so the pitch is mantained.
Later , got to go,
RICH