@AC and about google being so much dumber than he is
I would venture an explanation, just to show there might be reasons you never stopped to think about. It may well be a stupid explanation, as I know nothing about the topic, but nonetheless it should remind you that if *I* am clueless on this, maybe if you stopped being full of it, you could start considering the fact google may have reasons you haven't thought of.
Home->DNS server-> main Google servers->modules.google.com servers
Home->DNS Server->gmodules.com servers
Do you see a difference? Yes, one less table to check to route the traffic, one less server to go through, possibly one less rerouting to the other side of the world to do.
Basically, ask yourself why the second one is good enough for google ads (which are not, I would guess, labeled googleads.anonymouscoward.com when they're seen on your website). Might it be because it would be pointless as you would then have to route the request to another completely different place in the world (google servers instead of your own) and wait for the reply (more traffic, more delay)?
Assuming google has sufficient traffic to locate parts of it to specific places in its portfolio of server farms (do you want to declare that you think they don't???), then targeting gmodules.com directly rather than google.com which then reroutes to some other server far, far, far away saves time and traffic.
What makes you think the data from gwhatever.com necessarily could be efficiently recovered from the address to which the DNS servers route basic google.com traffic?
Because you're not google, then you're technically justified having the information on your website sourced from two different physical locations (one for google's ads, one for your own content), but because google is a single legal entity, then they're not? Even though as a single company, they have to process data in tens of different server farms in different places in the world?
Excuse me, but it seems more likely to me that they know exactly what they're doing based on a very deep understanding of DNS and that you just didn't stop to think before posting.
I would guess that when the traffic is sent to different namedomains, then it's exactly because google may need to split the recovery of the information of the page between very different places and thus need the DNS servers to know that to avoid some back and forth across continents.
Disclaimer: part of my example of explanation may not be relevant since I don't know much about DNS. Still, I think you get the idea and can now apply it correctly with your own understanding of DNS, which is so much better than google's