Why is there a huge difference between the outputs' of top command and the system monitor when both programs were run at the same time. Please find attached is the screen shot showing the huge difference:-
Is this a bug?
If you are referring to RAM usage, it is because system monitor does not include buffers and caching, just actual Ram used to run applications, top is slightly confusing because it does not obviously show disk usage after buffers / caching. Try using the following command :
See the image below, and note that the second line of free -m indicates available Ram, whilst the first line indicates actual Ram usage including disk caching.
I would recommend installing and using