When dealing with “signal level” (the level of the audio in a track), the more negative the number of dB, the quieter (lower level) it is. -37.9 dB is 17 dB lower than -20.9.
Looking at your first screenshot, it looks like the “noise” level immediately after the highlighted section is probably higher than -37.9 dB, and will therefore not be recognised as “silence” but as a continuation of the sound.