I might see an ad today, think about it for a week, then come back and buy the product later. I might keep buying it for years afterwards. The ad is often just one step in the process that gets somebody to buy.
I think of how I got turned on to Monster Energy drinks because I saw them at the store and liked the product and the portion size compared to Red Bull. A few years later Monster started spending heavily on sponsorships and other marketing but by that point I had lost interest and moved on.
> 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.
For brand lift campaigns though, without enough historical sales numbers, I can see how it’s harder to tell if your ad money is being well spent.