To explain how this happens will take several pages of notes on the complexities of ad serving that are the exact opposite of "not clever enough".
A summary is that your adblocking may have an impact on syncing stats, your purchases are not unique enough to matter for that category, serving some ad is better than serving no ad, and it maybe better to show you that impression however worthless than to bother with something else considering the auction dynamics at that point in time.
Do you think that's true when it comes to Amazon's recommendations, which I'm assuming are largely based on activity on Amazon itself? My adblockers don't keep Amazon from tracking things I do on their site while logged in, nor from showing me product recommendations.
Actually before I was ad blocking, two large local electronics stores did the same stupidity. I bought some expensive electronic toy, then was followed for a week by google ads for the exact same electronic toy. Which i had just bought. And was expensive enough to not buy another. And the ads were for the EXACT SAME STORE i had just bought it from.
I repeat, that was before I began blocking ads. For a while I was on the 'let the web sites make some money' side. Then the web sites became unreadable with ads on so it's uBlock Origin now.
That's what I described about the ecommerce system being different than the ad network. Products usually have their own pages where you can easily be added to a 'interested' segment.
It's very hard to remove you from that segment on a consolidated check out page that would need some backend mapping with the products you bought and the removal pixels for each product and ad network, along with lots of other issues. It's just not that simple as you may think to keep things in sync.
So they're okay with me considering them stupid? :)
As the customer, I don't care how hard or easy it is. I can imagine how it would be implemented, but 99% of the world population can't.
Note that they never gave me ads for stuff i only looked at, only for stuff that went through my shopping cart. If it's already there, I guess it makes figuring out if i bought it or not easier?
A summary is that your adblocking may have an impact on syncing stats, your purchases are not unique enough to matter for that category, serving some ad is better than serving no ad, and it maybe better to show you that impression however worthless than to bother with something else considering the auction dynamics at that point in time.