Because the FPV (just like the VSI indication) cannot tell from IRS & accelerometer information alone when the aircraft has a zero rate of climb
I (originally) thought the beauty of the FPV was that it
was independent of the baro system.
As far as I know, the
attitude of the aircraft is not corrected by baro (only by gravity over the longer term). From this, it seems I falsely deduced that the FPV circuits computed deviations from this frame of inertial reference.
Anyway, thanks for the insight (but I could do without the rolling eye icons, thanks)
Regards
NSEU