My home network & cellular hotspot runs in IPv6-mostly configuration and, in order for certain applications that doesn't work with the current Internet protocol (for some reason), I installed clatd in my system.
When netspeed is set to monitor "Default" interface, it monitors the clatd interface instead of the actual network interface I am using, rather Wi-Fi or Ethernet. And as I work at different places and have to switch connections quite fequently, I always get surprised by netspeed showing close to no traffic when I am actually doing lots of data transfers and benchmarking links. I know, seems not a big deal doing 2 or 3 clicks to set the interface I'm using at that moment but I always forget to set that back and I understand the "Default" option should cover that.