Thank you very much!
Oh no, it’s DSA again. 
I installed ethtool (opkg update , opkg install ethtool) and tried your suggestion.
I can confirm that in_good_octets and out_octets contain the desired values.
Within the next days I will try to adapt the script from LED color based on bandwidth usage to use ethtool instead of /proc/net/dev.
I was wondering - maybe you (or someone else) know - how TurrisOS or OpenWrt address this issue (if at all). As far as I know, the Realtime Graphs are a default feature in OpenWrt. Apparently (I did not research in this) those access /proc/net/dev, too, as they do not indicate traffic between switch ports.