Where does nload get the bandwidth data from?

tl;dr The bandwidth usage recorded by nload (home page) is higher than what's provided by tcpdump (home page).

There seems to be a difference in bandwidth usage when I run tcpdump -e -w - | pv -a > /dev/null as opposed to running nload.

For example, I saw a bandwidth usage of 150kbps using nload but only 30kbps using tcpdump. To rule out pv, I've done the following:

# open a tmux session where tcpdump dumps all traffic into a file for 60 seconds
$ tmux new-session -d -s tcpdump_60secs 'tcpdump -e -w tempfile' && sleep 60 && tmux kill-server

$ wc -c tempfile
# divide by 60, resulting in bytes per second

Any pointers to how nload gets its bandwidth statistics will be appreciated.

Replicated on a clean Ubuntu 18.04 install, on a LXC proxmox container, tested with nload build using commit 8f92dc0.