As far as I can tell, Activity Monitor and footprint(1) both grab the phys_footprint field from proc_pid_rusage. I have not seem them diverge yet, so if there is more to this that I am missing I (as the author of a system monitoring tool of this sort) would be interested in hearing about it.
See "there’s more columns than that". The single process "memory" column does match but AM doesn't show coalition (multiple process) memory totals, the compressed column is an estimate, footprint --vmObjectDirty also exists and is a valid way to look at things, etc.
Ah, I see what you mean now; yes, it's not reliable in the sense that there are ways to measure it that look a little harder. That being said, I'm not sure I'd classify it as "inaccurate"–it has, in my experience at least, been a fairly good first approximation of memory usage.
