It could be that the bias means that the camera chose the best (closest) shutter setting (you were in Aperture priority) and even with the closest shutter, the exposure is still -1/3 away from what the built-in metering decided.
Its in AV mode. I notice in breezebrower the exposure bias is not there. But the exif in show 1/3. Only this photo has this problem. The rest of my photo shows 1/2 cos I always set to +1/2 by default. Funny. :dunno: