I was testing my airspeed sensor installation by tapping my finger on the tube as advised. The airspeed increased and the variance decreased as expected. But each time I put my finger on the tube, the artificial horizon showed that the nose of the plane dipped, even though it was firmly on the table. This seems to happen with and without GPS lock, with and without GPS connected to APM. But it does not happen if I uncheck the box "use airspeed". Then the airspeed reading behaves as expected but no false estimation of pitch. It happens in manual and stabilize modes, I have not tried other modes.
What could be the reason?