Had the same thoughts as Shaddows before i read his post.
One thing i've noticed a few times now - when i've accidently placed noindex on some crucial pages - traffic dropped off... i spotted the drop, figured out the mistake, and BANG! A whole lot of traffic, more then usual, that then dies down to normal levels.
Shaping is real. I have sites with 40 or so popular pages, and a get a almost flatline (sam each day) of traffic to the entire site.
Humans are not that predictable!