Confused by the trailer weight versus gravity and reality

[Walnuts]

Hi All,
Hoping someone on here can help me, because this maths has me confused.

My TT is a 2021 318BR with a dry weight (from the sticker) of 7686 lbs, and GVWR of 9700 lbs.

Last weekend on the way to the campsite, decided to hit the Cat Scale. Propane tanks, 2 x grey, black tank and fresh water tanks were all full. A single battery on the TT.
The Cat scale said; 8320 lbs. I'll accept that, we tend to travel light with the accessories etc. there's only 2 adults and a 6 year old child.

Coming back from the camping weekend...I'd dumped all the tanks to empty. Had only used enough propane for the water heater (so not much).
Hit the same Cat Scale and the TT weight recorded at 7540 lb.

Now here's the issue...how can the weight of my TT (with some chairs, beers, onboard) be less than the weight from the factory?

I don't understand it.
Am I to believe that either the factory scale or the newly installed Cat scale is not calibrated?

For clarity, the axle weights on the TV were approx the same both going and returning for the trip.
Being a little OCD, I tend to pack things in the same places and conditions every time. <--- Nerd Alert!!!

Thank you in advance for any input, its appreciated.