The "N" rated tyre system is a Porsche approval system. They start with N0, then as subsequent upgrades of tyres are replaced they have the N1, N2 etc...
I would assume your car came with regular Cup 2's, the fronts have worn and been replaced, as the rears wear less these new N0 tyres will have been put on the rear with the base C2's on the front to wear them out sooner.... You could check production dates on the sidewall to verify this.