Why does level on panadapter drop when you zoom?

Check out Steve's explanation here:
https://community.flexradio.com/flexradio/topics/noise_floor_chang

The pan adapter gets its display information from the FFT, which is a collection of about 4096 simple receivers. Each of these simple receivers does the same thing, They are each tuned to a fixed frequency controlled by the pan adapter and each receiver responds to that frequency through a very narrow filter (about 2 to 50Hz wide ). The collection of these receivers cover the frequency span of the pan adapter.

When you ZOOM OUT, the frequency span of the pan adapter is doubled. Consequently the filters on all 4096 receivers have to be doubled in width so that they STILL cover the doubled frequency span of the pan adapter. When you ZOOM IN, the frequency span of the pan adapter is cut in half. and consequently the FFT receiver’s bandwidth is also cut in half. So what’s the point? Whenever you reduce receiver bandwidth in half, the noise reduces by 3dB, and doubling receiver bandwidth will increase the noise by 3dB. That is why ZOOMING changes the pan adapter’s noise floor height. IMO the pan adapter’s scale should indicate FFT bandwidth or have a 2nd scale that is calibrated with the active slice receiver’s bandwidth.

A final point. The pan adapter’s noise floor level or height has absolutely nothing to do with a slice receiver’s noise floor. Usually the slice receiver’s bandwidth is wider, especially phone bandwidths, then the pan adapter’s FFT receivers and that’s why the noise registering on the S-meter is always higher then the pan adapter’s noise floor! Aside from the fact that the signal source is same, slice receivers and the pan adapter hosting them, are independent.