One thing to keep in mind is that bandwidth quality can vary from host to host. Not all IP transit providers (the Internet backbone companies hosts buy bandwidth from) are equal. Some hosts may use lower quality IP transit providers which will decrease their bandwidth costs, allowing them to offer you more bandwidth, but the bandwidth performance will not be as good. That could result in higher latency (slower ping times), packet loss, more frequent downtimes, etc., which will all have a negative effect on the performance of your site. Also, some hosts may only have one transit provider, which may decrease their costs but it also decreases reliability because if that one connection goes down, everything goes down. Good bandwidth isn't inexpensive. Much of the reason you see big bandwidth offers from hosts is because, like others have said, they are overselling.