Most companies don't serve their own ads. They sell an iframe reference in a particular kind of page to someone else who runs their own adserver software, so there's a detectable footprint for organisational reasons.
Quite often there's more than one level of resale; I've seen five and a friend who works in the business told me he's seen even more.
(Digression: I have also heard that Ghostery (.com) makes most of its money by telling companies what ads are on their web pages.)