You're not alone in your praise for this little machine. This has been my go-to pocket recorder for the past 10 years! The ergonomics are great & nothing currently manufactured comes anywhere close in terms of low-noise specs.
But interpreting the specifications in a meaningful way can get complex pretty quickly, so don't worry! The Avisoft figures are based on measurements of an actual sample & they're a useful comparison between devices as Raimund Specht has a clearly defined test procedure which he follows closely. The (reputable) manufacturers specs tend to be worst case. i.e. every sample will meet the spec. So there will always be a discrepancy between the two sets of figures.
As a simple rule, GLouie has already said very succinctly, "low" sensitivity is for loud sounds, eg, speech, music, etc. "high" is for when you need max gain for very quiet sounds.
I can't really understand the implications on recording quality of these settings - as in which provides a cleaner result?
Is "Low" attenuating the signal or is "High" boosting it?
In terms of quality & cleanliness of signal, it depends on what you want to record.
Ants walking on a leaf - you want lowest noise, max amplification & you're not worried about overload, so select "High" sensitivity and use your most sensitive mics (EM172).
Music - Noise isn't an issue, but possible overload is. So use "low" sensitivity & your least sensitive mics (CS10EM). Adjust the level control so that the levels are peaking at around -12 to -6dB
Both settings are amplifying the signal. It's just that the "hi" setting gives you 20dB more gain, Think of it like a gear change.