[PCR-1000] Pcr1000 bandwidth

[Dale Ireland]

I use my pcr1000 to receive NOAA satellite images in 137-138Mhz. They
require a bandwidth around 35-40Khz. The pcr1000 does an OK job as it has a
50khz width.
If I select NFM 50khz I get a pretty good image but if I select WFM 50khz it
is much poorer as if the bandwidth is too narrow. Everyone who uses the
prc1000 for NOAA apt uses NFM. 
Why would NFM 50khz be any different than WFM 50khz? What else is changed?
Dale