More Baby Blankets and a Question

Here is the latest batch of baby blankets that I will be donating to the local group that distributes them to new mothers in the community. All the blankets but one are crocheted, based on either individual granny squares or one big square. The last one is knitted in the Old Shale pattern.

Here is my question. If you don't crochet, just skip this odd little discussion. The middle of the blankets done in one big square is wonky--not squared up. I've checked the Internet and see that other blankets made this way have the same appearance. When I first started crocheting, I thought that this happened due to my own error--I didn't understand how to turn the corners on the square (missed reading that part of the directions!)--so my "squares" were actually spirals. Now that I have corrected that error, each round (actually square, in this case) is completed with squared corners before a new round (square) is started, so I don't understand why the center squares look so crooked.

If you have the answer, please leave it in the comments. I would love to solve this mystery!