Operating systems calibre runs on
 

Looking at https://calibre-ebook.com/dynamic/calibre-usage
I noticed that it lists:
* win 82.3%:
* osx 14.3%
* oth 3.4%

Converting that to available downloads:

Windows:
* 32 bit msi;
* 64 bit msi;
* portable exe;

OS X
* dmg

Linux:
* x86_64;
* i686;

Source Code:
* tar;

Is there any reason for saying "Other", rather than "Linux"?

calibre runs on bsds too, though not officially supported there. I believe its been ported to haiku as well.

Thanks.
I didn't know about the unofficial ports.

Also been ported to ARM for the Raspberry PI. Basically the user has to compile the Linux source for ARM. No doubt Gentoo Linux users like to compile their own i686 or x86-64 source too!
Linux has the vast bulk of server and embedded system installs, but the inertia for people to stay with Windows or Mac on the desktop is high because of past MS Propaganda since 1998 and your desktop comes with Windows or Mac OS already.