I have always assumed that if "all" is specified in the protocol list, a modem will try the most "advanced" protocol first and if synchronisation is not achieved will then work down the list in a stepwise fashion.
I.e. VDSL2, ADSL2+, ADSL2, ADSL (or, if you prefer it, G.993.2, G.992.5, G.992.3, G.992.1).