UPS Shipping weights differ from USPS

I have both UPS and USPS on my shopping cart for customers to choose shipping preference. However, UPS shows a heavier weight why creating a higher costs if choosing UPS? Is there a way for me to correct that or does it need correcting elsewhere if so who do I contact? I talked with UPS and they said its a 3rd party issue. Thanks for the assistance.

I am not following you on this ...

What is the weight that you are testing and what does it weight for UPS and what does it weight for USPS ...

Are you using UPS that comes with Zen Cart or are you using UPS XML Add On?

UPS I have enable UPS shipping. With USPS it has the USPS Web Tools regsitered. Server to use shows production.

An example to use would be USPS would show the weight being actual what I have calculated it to be such as 1.02 lbs and UPS would show that same item as 2.03 lbs. Thanks

I cannot reproduce this problem ... are you using v1.3.9h