Janey, you are right, I hadn't noticed it, but the directions say to trim the bottom left edge and the top right edge of the "shadow" fabric to give it that look. I would say that is optional, and I might not do it. My day yesterday was full of appts. and a birthday lunch for my DH, so I didn't pay much attention to that part of the pattern then or this morning, I was just looking at the picture but didn't notice the two corners. I'm sorry.
But all of that to say this....I like the look of the corners after I noticed them!