Disable AF-point bias on eval. metering 5D mk III??

Coming from a 5D mkII, I was caught a bit off guard when I did an informal corporate portrait shooting and my evaluative metering exposures were all over the place within the same series of shots.

Basically I was alternating between landscape and portrait mode, and depending on how I held the camera, often just moving it a little would change my exposure by 2 stops.

I didn't understand it at the time, but later realized that it was a very strong AF-point bias sometimes targeting a black turtleneck and sometimes metering off the light gray sky.

Normally with my 5D mkII I would exposure compensate on gut feeling on the first few frames, then verify by looking at the histogram, and then I'd be set.

With this AF-point bias I couldn't relax for a second and it distracted me.

Can I switch it off? Can't find anything in the manual or in the menus.

I'm focusing on the back of the camera with AF-ON, and part of the point of this is to separate metering from focus.