Well, as with anything in hosting or in life, what are the chances of a customer using 100% of what he's given? Assuming that on average the customers use 90% of what they're allocated (huge number), the host would still be able to safely "re"sell the remaining 10%. That could be 10% of pure profit, and from a business point of view, it doesn't make much sense not to take advantage of the situation (except if the marketing is focused around a "no overselling" policy).

This is of course an oversimplification, but the principle does apply.