More than just a box?

In fixing the mailbox, we discoverd that the shape of the mailbox bottom is not square. That is, it's not a rectangle with square angles. Rather, the mailbox is slightly tapered from front to back. Why is a mailbox tapered? We're not sure, but our theory is that it shifts the balance (center of gravity) of the mailbox forward by forcing you where to position it over it's "foundation" plank and hence how the weight is distributed down the post. Considering that most times when I get mail, the mailman has shoved everything to the back, I suppose that's a possible reason. However, I wonder whether the weight of mail is significant enough to deem such engineering into a simple box and pole. All I have to say is it's not my theory...