I too would like to see a side-by-side comparison but if we assume that the claim is true, one reason might be that NetBSD runs very few daemons after a default install. I run NetBSD on a handful of Raspberry Pi Zero machines, and it is really quite a surprise that they run as well as they do for a $5 computer. Your typical Raspbian install has a lot more going on after the default install.

> Your typical Raspbian install has a lot more going on after the default install.

I believe that this is true for just about all widely popular distributions. It's probably possible to set up Arch to have power draw similar to NetBSD, but you're going to have to know what you're doing and it's probably going to require more administration/attention to keep running smoothly than NetBSD does.

I had Arch installed on my RPi2s, but I didn’t like the need to update the RPi that frequently, wearing off the SD card. And later on, it became unsupported by Arch ARM. Now I use DietPi, and it’s my favourite Linux distribution (for an SBC server) so far. It has very small number of processes running too). So that’s why I’m curious whether it would be much different with NetBSD.

Do you use the built-in WiFi on any of the Raspberry Pis?

By coincidence I installed NetBSD last weekend on a Raspberry Pi. Never used it before, but it seemed very nice. I had some issues with sshd (most likely just me doing something stupid) and never got as far as experimenting with WiFi, but supposedly there is some support (unlike FreeBSD and OpenBSD that do not support WiFi on any Raspberry Pis?).

Not with any of the BSDs, although I have tried. Instead I either use a USB to GigE adapter or a Waveshare carrier board (for the Zeros). The built in NICs for the ones that have them usually work fine.

If you really need wireless you can buy a USB WiFi dongle, and since there is a lot of support a compatible one is restively easy to find.

I just measured it by myself with a domestic power usage tool.

That is fascinating, one of the reasons I hadn't done that is a concern about power management features. I'd read openbsd at least had them running flat out most of the time.