The suggestions are probably based on browsing history rather than on buying history. It's not that surprising; you likely browse for multiple items, which helps make inferences about what you might want to buy. You only buy a small fraction of the items you browse, much of them unrelated. Taking one of those points (your purchase) and guessing that it belongs to a cluster of points that shouldn't be suggestions is not obvious. Further, basing suggestions on purchases might give terrible results given that your purchases are mostly unrelated.