It could then possibly be that even though you have paid them in full, they have held off paying the airline/holiday company to keep the cash in their system to help their cash flow. as such, the contract price with the airline/tour operator can change as it is not fixed until the full remittance has been paid.
i.e. with airlines, once the full sum is paid to them, you have a binding contract at that price (with the exception of government imposed taxes) its the same as buying an item from a shop, the price agreed and paid is the price.