The fuel flow sensor only outputs pulses every X thousandths of a gallon, not voltage, so if the flow number reduced, there's only a few options:
1) The flow actually reduced.
2) The perceived flow reduced because of turbulence in the line, and the change in flow changed the way the sensor reacted.
3) You were flowing too high for the sensor and it's in a non-linear region. These sensors are good to 60GPH, so probably not an issue with a rotax.
4) You have a bad connection and the EMS is actually reading noise, and this noise changed when the RPM changed.
We don't have an install manual for the flowscan sensors. They are pretty limited in their info for aircraft installs due to liability.