Ask yourself if you subscribed to /r/tech or /r/technology or both or neither (or /r/pics, /r/pic etc., whatever you jam is) and you will have your answer.
A community name is just an address, both on Lemmy and on Reddit. It never mean that that address had the exclusive rights to a topic.
Yeah, this actually happens on Reddit a fair amount. Off the top of my head, there's /r/tearsofthekingdom and /r/totk which both have 100,000+ subs, and /r/nsfw_gif and /r/nsfw_gifs that have millions of subs.
EDIT: There's also /r/gaming and /r/games which was a notable early community split when people started complaining that /r/gaming had too many low-effort memes.