Although the pairs of images seem to be centered on approximately the same focus point, if you are using spot metering, any change of focus point can drastically change the exposure reading. I'm not familiar with Pany cameras, but you said you think you are using multi-point metering. If this is a large version of spot-metering, it would be less prone to drastic changes caused by the slight shift of focus point, but still, I think it might be the case of this which nobody has mentioned. The exposure bracketing seems more likely, however.
Multi-point metering on Panasonic is the same as ESP on Olympus. The metering is averaged across the frame.