UPS shipping rates differ from ones on their site

[Gorgeous537]

We have an account with UPS that gets us a discounted shipping price, but we want to retail shipping price to be offered in the cart. We've tried using a different account, but the API still gives a price that is lower than what the price calculator gives on their website. We also tried changing the shipping to dimensional shipping for a few large, but light, items, and the price in the shopsite cart was the same as when we went by weight only. Am I missing something? Why is the price different from on the UPS site and why doesn't dimensional shipping give a different price than weight only?