There have been (very) many questions, responses, and outright arguments about this subject. The signal level on the dB scale is the SUM of ALL signals within the FFT Bin, which depends on what part of the output you are referencing, some being: Panadapter: There are 4096 bins represented across the panadapter width, so the band width of EACH bin will be (roughly) the panadapter total width divided by 4096. The dB reading will be the sum of all signals within that width. That is why that value will decrease as you zoom in (frequencies per bin is reduced). Receive Bandwidth: On the S-Meter for EACH receiver the reading will be the sum of all signals within the receiver bandwidth, including noise. So for example, a CW signal that is just above the noise level, at 50hz filter width is will represent the signal level + a little bit of surrounding noise; widening the filter to 400hz will include more noise and the S-Meter will rise slightly; widening the filter to 2Khz will include much more noise and likely other signals as well raising the S-Meter much more as now the CW signal strength is much less than the sum of all the received noise. Conversely, if receiving a SSB signal then as you narrow the filter you will only be measuring part of the audio bandwidth and the S-Meter (or dB scale) will decrease as part of the original transmitted power is now within the receive bandwidth.
The learning curve may be steep but the basics are: The dB reading is always all the signals that are within the appropriate bandwidth (panadapter bin size or RX filter width) and will change as those functions are changed.