That is very similar to what I get because I have my browser set to enlarge things due to my poor eyesight. Here's how the banner looks on my screen:
However, the postings, etc. are centered properly. I have my browser set to "zoom" things at 143%, not the normal 100%. This is why I lose many things on the right side of the screen.
I also have an older 5:4 monitor, which is almost square. I am guessing folks with the newer widescreen (16:9) monitors see Dave's banner as being centered.