"Another is to group related websites together and run ads evenly across all of them. This is how most smaller ad networks work." I help design ad ops software

"Another is to group related websites together and run ads evenly across all of them. This is how most smaller ad networks work." I help design ad ops software for ad networks, and ads definitely don't "run evenly" across ad networks. Some campaigns are broken out into hundreds of parts, where a particular part will target a certain set of creatives at users/ad spaces with certain parameters. The parameters include browser, BT, time of day, connection speed, geo location, and more. It is actually quite rare to have a straight run of network campaign.