Thinking about it in slightly different terms:
If you have a two blade propeller in the 3 and 9 o'clock position and then pitch it nose up; then the down-going blade has its pitch increased and the up-going blade decreased.
This is because the aircraft pitch axis is parallel to the pitch change axis at this position.
The effect will reduce to zero as the blades reach 6 or 12 o'clock positions where only the apparent blade sweep is different.