I am not sure, I am weak on BTTF, but the pics were cool. It might just be an odd angle. It's possible that the driver's side door jamb overlaps the driver's seat. No clue. The link has eight more concept pics of different items.
I'm ashamed to say as much as I love BTTF i'm not well versed in much of this either. I just figured it would be the sketch showing what everything looks like without that seat in the way. At least that's what I'd do according to how our teachers wanted it in college. Those are some neat sketches, I always love looking at this stuff and half the time the stuff they didn't use was cooler than the stuff they went with.
The top concept is one of the final concepts done by Andrew Probert. It was done after Ron Cobb left the production. The middle one is one of the first sets of concept sketches done by Ron Cobb, who was the designer hired to design the DeLorean. The third is from the second film, it was done by John Bell, there is mention of this over in a new John Bell interview I did on BTTF.com - The Official Back to the Future™ News Source (its on the main page). The last is the second final Probert concept. There are more Probert designs, many were done before Ron Cobb came onto the production. I did a good interview with Probert years ago, because of the bttf.com crash, but it can be seen on the archive.net saved page here: An Interview with Production Designer Andrew Probert
Hope that helps with the backstory. Concept art is always so cool to see!
There is alot in there, but there is a ton of other stuff out there. Some concept stuff I have yet to see, the actual concepts for 1985A for example. Also there are a ton of concept art of the DeLorean that can only be seen in person if you catch Michael Scheffe's discussions. People at the We're Going Back event I believe saw stuff along with people at the BTTF reunion in Alabama.
I know there will be more interesting articles on bttf.com about the designing, building and filming of the movies. There are some cool details, so keep checking the site!