That seems to be a function of the number of items that people bring to those checkouts rather than anything else. All things being equal, it's always been my experience that the self-checkouts are slower, not faster.

Sure, but there's one line for 4-8 self checkouts which makes the line move faster even if everyone is a technophobe.

Usually because they do the correct thing and have one line for 6 machines, while every other line is for one particular cash register.