same reason you cover your cards when playing poker. Puts you in the driving seat for future negotiations. If clubs think you have pots of cash to throw around up goes the price. Look at the inflated fees man city have paid.
So why did we let the whole world know we paid 1.2m for Hume? and surey an undisclosed fee is then another way of saying you have pots load of cash, hence not wanting to tell everyone how much you've paid?
The fee Barnsley pay and Charton recieve will not be the same. Mixed in will be Charlton paying abit of Grays contract up, signing on wedge for us to make up for wage drop plus agents fee, not stright forward at all
All transfers fees would no doubt be undisclosed except that clubs like to advertise how much their new star player "is worth" to sell more season tickets.