No, that's not normal. The lid should sit flush on the lip within the middle cooking section. I've never heard of this problem before. You might try placing the lid on a surface you know to be level. If it doesn't rock, then you know it's the middle cooking section and not the lid.
Which ever piece is out of whack, call Weber at 800-446-1071 and they should send a replacement free of charge.
Regards,
Chris