Strange behavior of getNetworkStats()

[Disinterpreter]

I've decided to draw some information about my server.
When handling getNetworkStats() I get the strange behavior.
The data in table as bytesReceived, bytesSent, packets* is lost when the player leaves from the server

In the screenshot you can see how data is reseted to zero.
In normal practices the graph always should be increased.

It should be like this in normal case:

Why it's needed for me:
I want to get the derivative data that I can analyse per time (e.g 1min)
But when player leaves away we can see this:
(Strange peaks)