Both images are 1/350 at f/11 ISO 200. Both are 100% crops (1:1 view) in Lightroom. The images have identical post-processing. So what's the difference?
In the on the left the bridge is near the center of the frame, in the image on the right it's along the right edge. After extensive focus testing, this copy of the Zeiss 21mm produced results that were consistently soft in the background about 10% of the way into the frame. Here's the problem in a nutshell:
Camera and lens manufacturing tolerances may not be tight enough to produce consistently sharp results with wide angle lenses on high-resolution cameras like the Canon 5D Mark II.
I've now tested 3 copies of the Zeiss 21mm f/2.8 and 2 copies of the Canon 24mm TS-E II, and only 1 out of the 5 lenses was acceptably sharp across the frame. Unfortunately, the 1 lens that was sharp was a rental. Three of the lenses were soft on the left edge of the frame in the background at f/8-f/11, and the Zeiss above had the same issue but on the right side.
The Zeiss 21mm and Canon 24mm TS-E II are both extremely sharp lenses, which seems to exacerbate the problem. The mountains in some of the photos I took yesterday were very sharp everywhere except the right 10-15% of the frame. Because everything else is so sharp, this makes the out of focus area even more apparent.
If you enjoy shooting with wide angle lenses and make large prints, this may end up being a problem for you sooner or later with high resolution 35mm digital. If you're thinking about upgrading to a 5D Mark II, or buying a new wide angle lens, here's a very important article to read:
"This lens is soft" and other facts -- by Roger Cicala of LensRentals.com
The bottom line: buy from a vendor with a good return / exchange policy, and carefully test any lenses after purchase to make sure they're sharp on your camera body.