From the Timeline on the Media Thread, page 3, Su NOV 13, 11:58 am:
"officers describe a bloody crime scene, "especially on upper floors of the house" [37]"
In my opinion, by saying "especially" that indicates that the entire house was a bloody crime scene, but it was especially bad on the upper floors with only minimal evidence (blood) on the first floor.
I see two scenarios as to how the lower floor had blood: either the killer tracked it down or at least one of the surviving roommates went upstairs, saw the crime scene, and then tracked it down.
If the police seem very disturbed at what that crime scene looked like, I cannot imagine how very difficult it would be for a young college student to see the murder scene for her friends. It is so easy for me to believe that she could have gone back downstairs and fainted and this is what alerted friends to call 911.