I just found out that the PWM freq should be 25kHz and that the TACH signal gives 2 ticks per round.
That means: In the script I pasted earlier "rounds/s" were just ticks and the "RPM" were "twice the RPM".
After adjusting the PWM freq the corrected measurements for the Noctua NF A14:
duty_cycle: 170ns, stops
duty_cycle: 180ns, ticks: ~3, RPM: 90
duty_cycle: 190ns, ticks: ~4, RPM: 120
duty_cycle: 200ns, ticks: 8, RPM: 240
duty_cycle: 250ns, ticks: 16, RPM: 480
duty_cycle: 300ns, ticks: 22, RPM: 660
duty_cycle: 350ns, ticks: 34, RPM: 1020
duty_cycle: 400ns, ticks: 40, RPM: 1200
duty_cycle: 500ns, ticks: 47, RPM: 1400
duty_cycle: 800ns, ticks: 36, RPM: 990 [*]
duty_cycle: 2000ns, ticks: 24, RPM: 720 [*]
duty_cycle: 2000ns, ticks: 26, RPM: 780 [*]
duty_cycle: 4000ns, ticks: 40, RPM: 1200 [*]
[*] fan is faster, but ticks aren't consistent.