580ex2 and color information

I recently ran into a situation where the color of two photos of the same subject with slightly different frames look different. I had one 580ex2 off camera and set to manual output power (1/16). My camera (5d2) is set to M exposure 1/60s f2.8 with WB set to 4000K. The scene is two people speaking at a podium, with window as the background, at night. First shot with the subject right in the middle of the frame, the photo color looks normal (seem to match 4000K wb that I set). Second shot with the subject on the left side of the frame, photo look a lot cooler (~2800K). Does anyone know why? From google search, one article indicates 580ex2 passes color info to the camera. What the camera does with that info was not mentioned.

Hi, I was waiting in case others had better info . . .

There were 'white papers' around the introduction of ETTL II but they didn't throw any more light than 'passes color info to the camera'. It wasn't clear whether or not that applied under both Evaluative for flash and Average for flash or just the former.

How much ambient was contributing, how far away were you, what was the zoom setting on the flash?

Since the podium was static, I doubt this was your problem. However, I have found when shooting in big rooms, the color temperature of the crappy ambient lighting changes considerably when the subject only moves a few feet.

What type of lighting was in the venue? Fluorescent or metal halide overhead lighting has a lot of color shift and you have to overpower that with flash to get consistent color temps.