FedEx keeps adding weight (and charging more) for my shipments

In the past 2 years of trading, I've shipped approximately 70-80 boxes. Forty-five are via confirmed trades, with a few dozen others to trading partners. With the exception of 2 boxes that have been shipped via FedEx, all others have been shipped via UPS. My preference is to ship via UPS but the two that I shipped via FedEx were due to the traders' requests that I use that service for a unique circumstance. One of those two FedEx boxes went out yesterday (with the other one going out sometime in the middle of last year).

In each of the two circumstances where I shipped FedEx, I've entered the dimensions and weight of the box and dropped of the box at my employer's shipping and receiving department with the label already affixed. My employer's shipping and receiving department is also the same place where I drop off all my UPS boxes in the same fashion (label affixed after weight and dimensions were input). In both cases with FedEx, the weight I input was increased and I was charged extra for shipping. In none of my other 70-80 packages with UPS has that occurred.

The likelihood that I've measured incorrectly (both on the low side) for only the two boxes I've shipped with FedEx and none with the UPS is very low. I am a chemist that works in a R&D facility with dozens of ISO certified balances. I use a variety of balances daily, from sensitive analytical balances going to 10,000th of a gram to balances that go up to 2,000lbs limit which we use to send material to NASA. For getting the weight of all my boxes that I ship, I use the balance that was audited by NASA and is used to determine the amounts of materials we ship to them.

TL;DR, In summary, has anyone experienced this with FedEx (or even UPS for that matter) of adding more weight to a box and charging more than original invoice? Do I have a FedEx employee who is using a malfunctioning balance for measurement, or is he/she using their balance incorrectly? Or is there something in how FedEx charges that I am unfamiliar with and this practice is perfectly within their policy?

 

In the first instance, I entered 15lbs and was charged for 23lbs. The most recent instance (yesterday), I entered 8lbs and was charged for 11.5lbs.

I also round up to the nearest lb when creating labels just to make sure I'm not going to incur extra charge or create an issue with the shipment.

 

This would make sense. I don't use FedEx
specific boxes so maybe that's the reason.