It’s a fact that resharpening taps changes the tap size or pitch diameter. There are 2 features that effect tap / pitch diameter size.
1. Back Taper begins at the first full thread (largest diameter) and reduces toward the shank at .0005 / .001″ pitch diameter reduction per inch. When regrinding the lead, (as stated in lead grinding) this usually requires shortening the tap 2-3 threads. When doing this the pitch diameter is reduced approximately 1 GH factor or about .0005″.
2. Pitch diameter relief is cam relief ground when the threads are ground. This means that radially from the cutting edge toward the heel of the tap, the pitch diameter reduces. When grinding the flute face, narrowing the land, the effective pitch is reduced slightly because the cutting edge is farther back into the thread relief. (graphic from lead grind page here)
The net effect of resharpening a tap will reduce the pitch diameter of a tap approximately 1 GH or .0005″, and yield a tap in like new cutting condition.