That's just because Reddit lets people see where someone posts. On Matrix and Discord, you can use the same identity to join two different rooms, and only people that are in the same two rooms will know that you're there. It's always cute serendipity when I join a different Matrix room or Discord guild and find someone I know from one of the others. Quite charming. Reddit's public-timeline-by-default is what encourages the crazy stalking and cross-sub banning behavior, but its norms come from a time when Reddit and the web was a lot smaller.