I knew I had read an explanation of this somewhere and I couldn't stop until I had located it.
See "The Imagineering Field Guide to the Magic Kingdom" by The Imagineers, on the bottom of page 24.
Here's the full text:
"This forced perspective, combined with the depth of the hub beyond the end of the street, opens up a vast and exhilarating vista to the Guest entering the Park for a day of adventure. Conversely, the full-sized buildings around Town Square and the lesser depth of the Square itself conspire to make the same path appear shorter to the same weary Guest trudging toward the exit at the end of the day. Think of it as an optical illusion for your feet."
The information about the actual vanishing points may have come from one of the tour guides on either 'Keys to the Kingdom' or 'Backstage Magic', I can't recall which one.