It has simply been a convention for a few decades or more to represent rotor or shaft speeds by N. When there are multiple shafts, then N1, N2 etc are used. Sometimes Ng (gas generator) and Nf (free turbine) are seen, or Nl (low pressure) and Nh (high pressure) - different customers you know.
The 100% reference speeds are purely arbitrary, sometimes based on poor guesses before the machine is first fired up. Thus some engines run at 90% at TO, others run 115-120% at TO. Sorry 'bout that.
