hp8901a manual

Hi

I am checking the residual FM on a free running osc design for a VCO (103MHz) in various bandwidths. I get strange readings on the HP

50Hz-200kHz (12Hz rms)
50Hz-20kHz (6Hz rms)
50Hz-15kHz (3Hz rms)
50Hz-3kHz (2Hz rms)
300Hz-3kHz (-8Hz rms) ?
300Hz-15kHz (-7Hz rms) ?
300Hz-20kHz (-1Hz rms) ?
300Hz-200kHz(+6Hz rms) ?

Why the negative sign and higher readings in the lower BW's? I don't find anything in the manual about this. Also I would expect the reading to be lower in the 300-3k than the 50-3kHz. The Res FM of the HP itself is less than 0.5Hz at this frequency according to spec. The green results agrees with a phase-noise measurement on the Spec A when converted. Maybe the -sign is a underflow indication?

Thanks E