Why is the memory usage reported by "ps" significantly different from that reported by "free"?

Issue

• The sum of the RSS values reported by ps is significantly lower than the amount of main memory used as reported by free.

For example:

$ ps aux | grep -v "USER" | awk 'BEGIN{sum=0}{sum+=$6}END{print sum}'
387056

$ free
             total       used       free     shared    buffers     cached
Mem:       2070740    1956740     114000          0     471584     863064
-/+ buffers/cache:     622092    1448648
Swap:      4192924        384    4192540

In this environment, the sum of RSS in ps is 387056kB. free shows used of -/+ buffers/cache is 622092kB. What is using the difference of 235036 kB?

• The total memory usage reported by ps aux is higher than the total physical memory.