With a dedicated server they're counting on you just wanting the server and not using close to 1000GB of bandwidth. With colocation if you ask for 1000GB they're assuming you'll actually use the full 1000GB. It's all about how much they can oversell, etc.
Also it's because the companies tend to reduce pricing for higher commits. Such as you can buy a 10 mbps 1/2 rack for about $700/mo. but when you order 10 mbps it's not $7000/mo. You can get it for cheaper with more rackspace. But also it has to do with overselling and such. Take this for example. If you buy 1/2 rack with 3000 GB bandwidth. You can then use aggregation so that if one server does 500 GB, another can do 1500. And another 1000. It's where a dedicated server you can only have 3 servers max to do the 1000 GB or else you're going to pay more. Basically what im saying is that with a rack you can milk more bandwidth out of it because you can put more and powerful servers on it to fully utilize it.