Normally the green bar is for uplink and the blue on top of it is for downlink, but I noticed that sometimes their roles are swapped for a short duration. It usually does this when there is a burst of high uplink traffic, and next time there is traffic, it's normal again. If I have multiple network monitor applets running, they all do this together. Reproducible: Always
My initial observation was wrong. The problem was that the two graphs were scaled independently, and a small traffic in one direction could be shown as much larger than big traffic in the other direction. Anyway, this doesn't seem to happen anymore. Closing.