MacOS – How to see total memory usage per app

Most of the difference was marked probably memory marked as inactive. Any app can still use this memory, so it's not marked as used by some process.
For more info about inactive memory, see this thread.

Wired memory usually makes up a much smaller portion. This is memory used by the OS; it isn't available to your apps.

For more info, see the Apple article Reading System Memory Usage in Activity Monitor.

You'll want sa and accton commands to dump accounting information to a file.

Here's an article that's a bit old. The perl script at the end works well - look out for * characters in the shell - those might need to get filtered or the input sanitized if the perl script doesn't show you totals.