How different modems measure attenuation can get complicated because there are at least 3 different main methods used to measure attenuation.
1) Insertion or Loop Loss - Method usually used by the line provider (ie Openreach) of the attenuation figure at a fixed frequency such as 300kHz as it gives a good indication of the loop length for estimating capacity of line speed. This attenuation figure should always remain fairly static regardless of the technology in use - barring any changes to physical condition of the copper line.
2) Line Attenuation. - Measured during initialisation of the line on all available tones in the band plan(s).
3) Signal Attenuation. - Can be continually measured during show time, but only monitors tones actually in use.
See
What is the difference between Line Attenuation and Signal Attenuation? for more information on the last two.
We don't always know which method any particular modem is using.
----
As far as the Zyxel goes, it does pretty well at giving the breakdown of both Line and Signal Attenuation per band, but you can only get these by telnetting into the CLI rather than the WebGUI.
From my observations I would hazard a guess that what they call 'Total Attenuation' is the aggregate figure of the Line Attenuation taken during sync.
The reason I say that is because I've never observed that figure changing during showtime. If anyone has ever seen that figure change during showtime, then the conclusion would be it is aggregate of the Signal attenuation figures.