It's probably one of those averages things whereby they bank on a certain percentage of users using more storage than what $50 a year should get you, some using less and most somewhere in the middle. Also, they could be using smart backup technology to only backup unique data that isn't already on their system. Microsoft's new Home Server does this by looking at bit level information on the drives it is backing up on each "client" machine. That way, you don't have duplicate copies of the exact same files for each client machine and thus save a ton of space.
They're also doing a few things to try and manage higher storage needs. From one of their FAQs:
"To keep users with very high speed Internet connections from hogging all our bandwidth and storage, we limit backups to .5GB per day once you have sent us 50GB of data. So if your initial backup is, say 40GB, it should run at full speed (2-3GB per day) until it is complete. If you add 1 GB of new data each day, after 10 days you'll have 50GB in your backup. After another 100 days, you'll have 100GB in your backup. There is no size limit. But you can only send us .5GB of new data each day once you have passed the 50GB threshold.
Carbonite will soon have a 'Power User' version that will allow full-speed backups of up to 200GB of data or more."
So, while the storage limit is still unlimited, the time required to backup larger systems will deter some of those users. I'd guess that the "Power User" version will be at a much higher price.
For people looking for good onsite backup solutions, I like what's coming later this year from both MS and Apple. Time Machine technology in Apple's upcoming Leopard OS looks very cool and includes the ability to backup to networked shares. It not only offers straight backups, but also includes multiple versions of saved files so you can go back to an old version of a single file if you need to. The trump card is the easy interface it uses to let you do that.
Vista offers that now with Shadow Copy. Later this year, Windows Home Server will offer a better networked backup solution for all Windows machines in the home. It's a smart, managed RAID NAS that isn't nearly as complex as a standard server box made to be used by home users.
Apple's Time Machine in Leopard:
http://www.apple.com/macosx/leopard/timemachine.html
MS Official Windows Home Server Blog:
http://blogs.technet.com/homeserver/default.aspx
Jeff