How can the Line attenuation vary?
Attenuation varies according to frequency, so there isn't just a single figure that describes the complete spectrum. If you look at the output from "--pbParams", it will give attenuation values for each band, for example.
So ... the modem has to aggregate all the attenuation values for all tones, in some way, to come up with a single value. That task probably leaves the modem firmware a little scope for doing things differently.
And, of course, the more complicated the job is, the more scope there is for bugs. Or subtle interpretations of specs. Or differences in receiver sensitivity. Or even just a dodgy connector pin on the modem.