Sometimes strange things happen with the weight.
When I load for example 2 guests onto my flight, the weight of these 2 people is listed as 300kg in the MCDU…
I sometimes had 400kg too.
So the average weight is not calculated with 75kg nor is it 100kg per person, but it seems to be a complete random value ranging from 10 to 250kg 
The calculation is very strange and seems to be in no way catered for charter or VIP flights.
(If one imagines two pilots and one stewardess added to the PAX weight the value becomes more plausible, but I have no idea how the weight of the flight guests in the Fenix is really calculated.)