At the end of any race the cars are weighed to make sure that they weigh a minimum of 798kg this is to ensure that there is no unfair advantage for running a car much lighter. Lighter cars go faster. It seems that at some point, Mercedes made an error with the weights on Georges car and at the end it was under this limit.
As other has said this could be down to any number of reasons and nothing official has been released.
I wonder if the lack of an in-lap to pick up marbles in Spa, combined with his long stint leaving him with little tread, contributed here. Could he have gained 1.5kg of rubber marbles during an in-lap?
A little under 400 grams per tire, it does sound like a lot but I wonder if that lack combined with the crazy long stint wearing down the tread contributed.