I know one of the dorms (it is Wyvern Hall, I checked), has two sims that will be part of one's playable household. Becca Clarke and Julia Wright. What's really happening is we're having our sim join the 'Best of Friends' household. And there's Cameron Fletcher at Foxbury.
If that's what you experienced, well, it's a design choice. It can be fun to have roommates, but if you chose the Drake Hall at Britechester or Tidal Tower at Foxbury, then it'll just be your sim.
If it was some other sim, I'd say it's a bug.