How come I sometimes see posts from one instance I am logged into, but from another instance I am also logged into (title@lemmy.world via sh.itjust.works)?

I just noticed that even though I was looking at and logged in through my Lemmy.World account, I would sometimes be posting/commenting as another account on another instance. All these posts will say "via <instance name>" next to the original location of the post.

It doesn't make much sense to me that it will use an account for a separate instance when I already have one signed in for the origin post, but idk if this is maybe a stability thing to just keep things running smoothly? 🤷🏻‍♂️