I'll tell you why it is, they are betting you won't use it. Most moderately busy websites use about 5Gb a month, 10 Gb a month would be rather busy. So they can offer you as much as they want, because nobody is going to use it anyway. I signed up with a company that offerred huge bandwidth for a low price, but my server was choking when I got up to about 10 Gb a month of traffic.