Totally spitballing:

The house had a basement. The basement has a separate entrance, or immediately off the front. X and Y live in the basement. X and Y were out drinking on Friday night and stumbled home. The 4 upstairs had not been seen since Friday night. One went shopping for late night snacks at 10pm. They logged phone/ laptop activity until 11PM. X and Y got home around midnight and nothing seemed off.

They immediately go to their basement rooms, and don't talk to anyone or know if anyone was alive. They thought everyone was sleeping and went to bed.

They wake up, get food and take off for work/ classes and don't talk to anyone upstairs and go about their day.

Police believe they were home, but due to the time of death being estimated anywhere from 11-1AM the roommates might've gotten home after the murders and just not seen any blood based on how they enter/ exit the house.

There are reasons they may not fully know who was home, and the roommates may not have seen any blood after the murders.

^ Above are not specific details, just how someone might believe a person to be home and not have actually heard or seen a thing.