I think it's how they word it. So far example if a fee is £5m plus a potential £5m in add ons (for making so many appearances, for getting an international call up or for the signing team winning the league/Champions League). The add ons can really add up and Celtic then give off the record press briefings saying the transfer fee 'could be' £10m.
On top of that they also often add on the players wages. So if the player signs a 4 year deal earning £2m a year, they can say the total deal is £18m (up to £10m in transfer fee plus £8m wages).
They're not necessarily telling lies, but their over-inflating the actual fee that they get (or fee they are guaranteed to get). And we've seen them do that big time with Dembele and Tierney and the press are only too willing to play along.