I invented a little story about that:
Brother Jenkins, one of the more senior members of imagineering, painted this art. However, he had an agenda. His son, Little Jenkins, worked on DCAII and designed a beautiful queue structure/boardwalk that would wrap around the base.
However, John Lasseter was also working on a project under the same budget. See, Carsland was John Lasseter's love child and it had to be perfect. He ran the numbers, and his personal budget wouldn't cover it all. He needed funding to do "immersive research" much like Joe Rohde's expeditions to Nepal to design Expedition Everest. He asked his good ole friend Bobby for some more dough, whom gladly obliged by shifting funds from Little Jenkins project.
So, the wrap-around pier was cut, and John was sent to some obscure town in Nevada in a private jet to find old license plates in the dump.
Years later, Brother Jenkins is assigned to work on a project under Bobby. As a tribute to his (now tragically deceased) son, he purposely adds the boardwalk pavilion to the plans he has given, and includes it in the art.
But Erin at the Disney Parks Blog saw what he did. She was the only one to notice, once the finished concept art was sent to her so she could blog about it. So, 10 minutes before she was scheduled to post the Pixar Pier details, she opened photoshop and put her graphic design minor to work. You gotta admit, it's a pretty good photoshop job, all things considered! So that is why there's that weird box around the base of the ferris wheel.