What is the difference between the memory usage report in System Monitor and the one by free?

I am using System Monitor 2.28.0. When I look in the Resources tab, I see a nice graph with for memory and swap. Memory is about 60% 2.3 GiB of 3.8. When I type the command free, I got :

             total       used       free     shared    buffers     cachedMem:       3994908    3962396      32512          0     100852    1477032-/+ buffers/cache:    2384512    1610396Swap:      8000328      28468    7971860

cat /proc/meminfo | grep MemFree give

MemFree:           34536 kB

The situation has remained the save for several minute. I started a lot of process with a script and the script is waiting for the free memory to get lower. According to what I am seeing in the Process tab (or with top), the number in System Monitor seem a lot closed to the total of the memory of the process that the one reported by free.

Thanks