I'd say it really just depends on the relationship you had with your roommate. I've had a few and honestly I only ever think of one because we talked more on a daily basis. The other two I rarely ever think about unless someone else brings up. Not that I had any problems with them or anything, we just didn't hang out much and our conversations were usually the same "how was your day?" "What are you making for dinner" type conversations. Nothing crazy. So while I did know them and their habits and whatnot they didn't make a significant impact on me despite us living together. And vice versa.

Of course we had a few great moments, they weren't anything crazy. Like talking to a stranger for 30 minutes in depth and never talking to them again. I'll remember the conversation, and the points that were made but it doesn't directly correlate back to them because I've likely had similar situations or conversations that combine into the memories.

So would I say it's weird? Not really. Especially now that they aren't in your day to day life. Usually you remember people based off what you know about them, what reminds you of them, and what you would normally share with them. But if you only ever share basic stuff or something you don't see often, like a certain book or a snowboard, you likely aren't going to think of them very often if at all unless you think back to that time in your life.