trying to understand something about nginx status: "writing" connections greater than "active" ones

So on this graph, can someone explain why the red line (writing connections) could go above the green line (active connections)?

Isn't the "active connections" the sum of Reading + Writing + Waiting ?

Here using nginx 1.12.2 nginx-stable-trusty from launchpad (Ubuntu 14.04)

deb http://ppa.launchpad.net/nginx/stable/ubuntu trusty main

Edit 1:

The graph reflects the figures returned by nginx status

Active connections: 215
server accepts handled requests
218305659 218305659 919815892
Reading: 0 Writing: 230 Waiting: 36