It's probably just a technical accounting update. Old assets are often kept valued at their buy price and not reevaluated every year to avoid taxes (Banque de France is not exempt from taxes). As they swap a type of gold by another and do a sell/buy action, the new gold is valued to current market price while the old one was valued in accounting at an old value.

They had a deficit last year, so they can probably avoid to pay tax this year by balancing last year loss with this year profit.