edit: Okay, after some testing, you are right about Cached Files not being part of Memory Used. But Cached Files doesn't always equal Physical Memory minus Memory Used either. In my iMac, it never does, not even close, but maybe that's because I have 40 GB of RAM. And as a computer engineer, I still think that Cached Files should be included in the Memory Used calculation, because everything I know as an engineer tells me that freeable memory, which is what Cached Files are, is still used memory until it has actually been freed, not free memory. Apparently, the engineers at Apple used to think the same way as me, as you can see by the previous design of the Activity Monitor below. This screenshot is from Mavericks, and as you can see, Cached Files used to be included in the Memory Used calculation and grouped in with App Memory, etc in that pane on the right. Not sure when Apple made the change to take it out of there, but it seems like some confused intern decided that freeable memory was the same as free memory. I guess Apple was trying to make it look like computers had more free memory than they actually did, because maybe some users were concerned that almost all of their physical memory was always being used on machines with lower amounts of memory.
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