Monitor ALL network traffic by app so far I used nettop and/or the activity monitor to observe the traffic used by each app but I noticed just now, that there is a huge difference between the sum of the per app traffic and the traffic which actually came through the network interface.

In this example the sum of the received traffic by app seems to be less than 5MB though the opened network interface monitor (right part of the screenshot) tells me, that actually 27.3MB came in. And this is the correct number as my mobile phone gives me the same figure for today's traffic.


So why is the sum of the traffic per app so much lower?

Regards
Simon
In the screen shot you are showing the Network usage numbers are excatly the same 27.3 MB. 

The individual network usage per application (coulomb) are current (active) numbers, which are much smaller and are not meant to be added manually to find out the total usage. OS X does that for you in the table at the bottom.

The Table at the bottom is showing the total usage for all applications and it is cumulative (for 7 days) or between restarts which ever comes first.

I did the test:

Data Usage before restart.



and Data usage AFTER restart: