Different 'categories' of hats have different prices. Some of these are less than 1.33, and so you will be charged that much metal respectively. The bot may also charge slightly more if it has very few hats available to sell. It may also be that a member of staff has changed the price of hats. This usually happens when there is a sudden demand for large numbers of hats (such as people wanting to craft them together in the wake of a new TF2 update). The price is usually set back to normal after a few days.

If you get offered less than 1.33 for a genuine hat then do not worry about it, it's supposed to be that way. Please, do not contact a staff member, make a thread in the forums or contact support about this.