> I might see an ad today, think about it for a week, then come back and buy the product later.

In general, unless a specific sale can be affirmatively linked to a user who has clicked an ad for a specific product, then it is excluded. Basically, the ad providers stats won't count people like you who clear cookies regularly, so the business benefits from those sales and the ad provider loses out.

So, in general, stats collection and reporting errors are in favor of the business selling goods.