XE1: spot metering bug or user error?

Hi all, I was just playing with my new XE1 and ran into a situation as below.

I wanted to shoot the mannequin off centre, properly exposed against the bright lamp to the right. I switched to manual focus mode, picked Spot Metering and moved the AF point to the torso of the mannequin and expected the torso to be properly exposed. However, the camera decided on 1/20, F8, ISO200 and the torso was way underexposed.

Without changing the lighting, I re-centered the AF point and moved the camera such that the center AF point was on the mannequin's torso, and this time the camera selected 1/1.2, F8, ISO200 and the torso was properly exposed.

I was kind of stumped as too why the off-centre AF gave me an exposure value different from the centre AF, considering I was using spot metering and that I filled the AF point with the mannequin's torso.

Am I misunderstanding spot metering, or is this a bug?