Memory pressure vs traditional
Memory pressure is a newer metric that Apple introduced as a simpler way to see how much stress is being placed on memory usage.
Traditional is an older metric that shows raw information about how much memory is being used.
We recommend using pressure, because it is a more modern metric that will give you a better indication of the load being placed on your Mac’s memory.
iStat Menus processes vs Activity Monitor
When showing per-app memory, iStat Menus combines child processes. Combining child processes shows a total that covers the full footprint of an app. This means if you’re comparing the figures to a single process in Activity Monitor, they will not match — to get the same calculation, you’ll have to add up the processes shown in Activity Monitor.
Activity Monitor shows a wide range of different memory values as different columns. The column that matches iStat Menus is called Real Memory. The real memory column can be shown by enabling Real Memory from the View › Columns menu in Activity Monitor. Real memory is the total amount of memory consumed by an application, including virtual pages.
At this point, macOS does not provide the information that would match Activity Monitor’s memory column — we only have access to real memory.
If you’d like to stop iStat Menus from combining processes, please disable “Combine Processes” from the Stats menu.