One picture says it all. My pawn was standing right where she is in the picture when the centipede started firing at the cat. Somehow, some of the bullets from the spread went around the corner and hit the naked rich explorer instead.
(http://i.imgur.com/JHPuwVW.jpg)
Just to compliment what mzbear has said, I tried this out myself.
(http://i.imgur.com/qAfbER8l.png?1)
I positioned the Sharon pawn to act as the target for the centipede, because Gussi in the position he's in will not engage the Centipede. So the Centipede shoots at Sharon, but since it has a spread of 2.9 the bullets end up falling around Gussi and in this instance end up actually hitting him. despite being behind a wall. Sharon also gets hit, but that's to be expected.
(http://i.imgur.com/VTJudsCl.png?1)
The game correctly understands that Gussi is not a valid target for the Centipede, but he is valid for the spread fire if the target is Sharon.