Some of those tags never existed on the main site. I put them there as a clue to new users what our chat is really about.
We've got one of the most active chats on the Stack. This means that people from any Stack Exchange site, when they look for chat rooms, will see ours near the top. We semi-regularly get people looking for video game chat, video game development chat, coding chat, mathematics chat, and just about anything else.
A few months ago, after our last batch of more joke-y tags got a little stale, I figured it couldn't hurt to put in some more "what we're about" cues.
(There really aren't many good tags for general chats: tags are supposed to be a lot more specific than a chat room for an entire Stack site. This leads to their tags being jokes, un-useful, or imaginary.)
They can be changed by room owners/moderators, but they're mostly non-functional descriptors anyway. There are only rare instances where chat tags are useful, so most chatrooms just either use them as jokes or general descriptors.
For instance, the Tavern on the Meta has the tags:
[bug] [crickets] [discussion] [excuses] [support] [waffles]
while in reality,
[excuses] don't link out to any useful tags on Meta Stack Exchange. Just a flavor thing.
Well, except waffles. Waffles are delicious. Though they're also pretty much just a flavor thing. But in a different way.