Activity Monitor is not a reliable way to inspect memory usage, it's optimized for speed of calculation. Use 'footprint' and 'zprint' or Xcode/Instruments instead.

Activity Monitor measures memory footprint.

Activity Monitor is almost just as useless as Task Manager in terms of reporting memory footprint. Not only will Apple's API constantly hide resources from native apps (like how Safari conveniently hides it's rendering processes), but MacOS's memory model is completely at odds with their measurement techniques. Either way, you're better off using top to measure your system's footprint, if anything.

Apple's APIs don't hide anything, programs on macOS just spawn subprocesses for reasons that mostly have to do with security. top(1) is not a very good way to measure footprint, footprint(1) is (and Activity Monitor uses the same APIs internally for its "Memory" column).

The way it does it is inaccurate and there’s more columns than that.

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.