Takeoff throttle not 100%

THROTTLE_NUDGE off
STICK_MIX off
on autolaunch, only get 94% throttle, even though THR_MAX and TKOFF_THR_MAX both set to 100%
with THROTTLE_NUDGE on and full throttle on transmitter, get 100% on takeoff

Arduplane 3.9.4 on Omnibus F4 Pro V3 Chibios
ideas?