"I can't really put my finger on why."
By the way, if you put us in the same room, we'll reflect that tension in our body language, regardless of our relationship to the other person. In another bizarre experiment, researchers showed a group some video clips of two people talking: one white guy and one guy who was obscured so the observers couldn't see what race he was. Test subjects could usually guess the race of the obscured guy just by watching the other one's body language.
The clips, by the way, were from TV shows where the characters were supposed to be equals and/or friendly with each other (that is, they weren't from cop shows where the white cop was arresting an obscured face wearing flamboyant pimp clothes). One of the shows that gave the "subconscious racist" response was Scrubs, for Christ's sake. These are real-life friends and colleagues playing the role of fake friends and colleagues. But put black and white together in the room, and on a subconscious level, things change.