No rush 
but another guess is the vertical axis is perhaps power and not power per Hz.
If you look at my simple self-noise test way back when:
You see noise was 2000 counts peak-to-peak (and very flat)
2000/56000 = .0357 Pa pk-pk
assume div by 2*sqrt(2) to get RMS
.0357/2.828 = .0126 Pa RMS
20*log(.0.0126) = -38 dB (relative to 1 Pa) {20 log gives us (Pascals)^2}
If you look at https://manual.raspberryshake.org/_downloads/SpecificationsforBoom_SnB.pdf
in the vicinity of 1-10 Hz, its pretty much -38 dB. Coincidence?
The bandwidth is almost 50 Hz which is 17 dB (relative to 1 Hz)
So the normalized sensitivity is going to be around
-38-17 = -55 dB/Hz
for comparison with other devices that show PSD, rather than just noise power as a function of frequency.
Due to the instrument frequency response, the sensitivity curve will slope upwards on the left side.
cheers