Ah, this is because cookies on ads that you see are all not tied to the conversion you made. In meaning, ads are broken down into clicks, impressions, and floodlights. The clicks and impressions generate the cookies where you wear "searching" for a car. The floodlight, which can also generate a cookie, happens when you land on the website and browse around for goods. When you make a purchase on the site, this leads to a conversion floodlight. 

Thus, when you are just browsing, Advertiser A, B, and C will see you looking. When you make the purchase on a website, only Advertiser C may know, but A and B will not know because you did not purchase off their ads. Thus, A and B will continue to advertise to you where C may remarket/retarget you.

Being you made the purchase likely at a dealer and not a website, A, B, and C prob still don't know you converted. They just have your clicks and impressions. I would imagine it's extremely hard to tie a car purchased offline to online ads on top of the fact advertisers do not tell other advertisers you purchased.