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I think the background softness in A region is due to lens softness. The plants are much closer, and so the effect is much reduced. This result is very similar to tests I've done with the 17-40L and various ultra wide primes. The 17-40L is very sharp in the centre and to the nearer edges (i.e. within a radius of 12mm of the centre), but softens towards the far edges and in the corners. For example, the 17-40L is very sharp in the centre, even a bit more than the Tokina AT-X Pro 17/3.5, and the AT-X is noticeably sharper in the corners. For most purposes, both lenses provide acceptable results. Pixel peeping is not one of those purposes.
Here's some examples. The first photo shows the full image, with the red rectangle showing the location of the first comparison set. The following three comparison sets all show 100% crops of the AT-X image on top and the 17-40L on the bottom. Photos taken at f/8, 1/125 sec, ISO 100, tripod, etc. The first set is near the centre, the second set is near the left edge, and the third is the extreme top left corner.