It's more complicated than what you are saying , what you witnessed is generally called behavioral targeting which is user specific generally . Sponsored search

It's more complicated than what you are saying , what you witnessed is generally called behavioral targeting which is user specific generally . Sponsored search generally is a solution which shows ads which have your keywords in it . Behavioral targeting ideally should show you stuff you might interested in buying or buy on a regular basis , if it's showing stuff that you already bought , it probably indicates a stale feedback loop between two systems