Actually, this was discussed towards the end of todays development meeting if you want to listen. It was my understanding that modules are also counted based upon 'update' requests, but it's a slightly different process (as I understood it).
The difference as I understood it, was that while core is aware of which modules are active on each site when that site checks in for core updates, that information is not tracked as part of this core request. Rather, the site checks with each contrib project directly for updates and each project keeps it's own statistics.
Nate was unable to give a really clear answer of the difference, which is why it would be helpful if anyone wants to dig in and document how this is all working.