View the memory inconsistency

I encountered a strange problem。
When I used “free -m” show the memory use status,It’s show have used 7GB memory,but use “ps -aux” show just used 4GB memory.
As you know ,when I used “free -m” show the memory status,I had Minus “buffers” and “cached”,caculated the used memory.
So I don’t know remain 3GB memory is used by which program.

How much RAM do you have in total? From what you’ve written I’m it sounds like 7GB, but 7GB seems is an unusual amount and if you have a total of 7GB then you have accounted for its use.

Does help?