"Seek the cause, and you will learn the end"
Ok, fake mystical insights aside...
I think it helps to understand the area and home. The area is a student district. Students like inexpensive rents and land lords like to maximize occupants via additional rooms or micro apartments while being with in building codes.
In this case, a home was expanded into a three level "split level" on a hill side. The lowest level (there are two ground floors), does not sit immediately below the other levels. Rather, it is behind of (or in front of) depending on the approach of the other levels. So, there would not have been noise coming though the ceiling per se.
It is also possible that an attacker might not realize there was another level to the house. Or, even if if he realized it, might conclude that it was a garage.
Factor the quirky lay out into the fact that the other two residents may of had a few to drink themselves, and I think its credible that they did not hear anything.