How is white ballance calculated on auto setting?

[kipp]

Hi everybody my question is as above "How does the camera (canon 7d) work out what to set the white ballance as on the auto setting?"
Do the metering modes affect this? And will low light affect it's ability to do this correctly?
I took some photo's at a mates wedding this weekend and it seemed to get it wrong (in my opinion). They were saved in raw so i can change them, i'm just trying to learn for next time.
thanks kipp