It must be because of users like me. I am a new Linux user but I have a theory about what the situation is.
One person installs Linux and signs up to a forum (Say Ubuntu forums). Then he starts to get tired of Ubuntu and switches to Mint. He doesn't see any reason to sign up to here because he thinks that any problem on any Debian based distro can be solved in the same way. There shouldn't be a big difference between Ubuntu and Mint. Considering that all Linux distros use the same kernel, this is a fair thought and he doesn't sign up to Mint forum.
And the other day, he wants to jump to OpenSuse. Because the main idea is the same, he doesn't feel he needs to sign for an OpenSuse forums. And, he can copy the error message and google it and find the solution! Some of Linux users don't even feel that they should sign up to a forum. Google the error and find the answer to the guy who had the same error
Besides, some people like to use forums in their native languages and if they have an active forum, he doesn't need to sign up to this forum.
Likewise, I am going to switch to Ubuntu. I may sign up to Ubuntu forums, but I don't need it (At least that's what most people think). Because I feel like I will be welcome here even if I don't use Mint, and most people know about Ubuntu too.
To sum up, the forum member count doesn't reflect the user count