Sigma 30mm: Is this a good copy?

I was checking out a used Sigma 30mm F1.4 lens. I noticed that the autofocus worked pretty well when I focus right in the middle. But when I focus using any of the other points in the 9 point focus selector on my 20D, the autofocus does a lousy job. This is with the aperture wide open at 1.4 by the way.

^ Here you can see that the autofocus is working very well when I focus right in the center

^ Here I tried to focus using the right most (3 o'clock position) AF point. The autofocus kind of got lost there.

^ Here I tried to focus using the top most (12 o'clock position) AF point. The lens ended up focusing closer to the middle.

So my question is- is this normal behavior for the Sigma 30mm? Or did I come across a(nother) bad copy of this lens?