Or because you just bought a new snowboard on-line (and won't need another for years), so now you've got snowboard ads everywhere, despite being in the group of people least likely to need a new snowboard anytime soon.

My shopping habits must be somewhat unique on a local amazon-esque site that 80% of the time when I view something I have already bought (i.e. to check some detail before it arrives, or sometimes if I want to order more - I buy a decent amount of hardware and fittings for DIY projects on there) it has a little carousel at the bottom saying "Others who looked at this item also looked at:" and it's a list of 8 things, 7 of which I know I've looked at recently, or even bought.