Amortisation refers specifically to intangible assets and is defined* as the deduction of capital expenses over a certain period - usually the life of any intangible asset.
Player registrations are deemed by accountants to be intangible assets. Their value is amortised over the duration of the initial contracts with the club. So, for example, if a player is bought for £10 million and on a four year contract, his expense would be recorded as £2.5 million per annum for each of the four years of his initial contract.
So if players X, Y and Z were bought in 2009 for a combined £50 million in transfer fees, each on five year contracts, Spurs would still, as far as the accounts are concerned, be paying a combined £10 million per annum for them in 2013.
So regardless of the fact that we might, on the face of it, only have spent £8 million on Parker and Coulibaly in the last financial year, we still had significant player trading costs carried forward from previous transfer windows. The money spent on Parker and Coulibaly, in fact, will have accounted for only a small proportion of the whole because their registrations too are subject to amortisation.
Amortisation also complicates the accounting of the sale of players. If we sell five players for a total of £50 million, it doesn't mean that we have made a £50 million profit. The amount of profit depends upon the valuation of the players' registrations in the accounts (and the valuation of the players' registrations is, itself, determined by how many years remain on each of the initial contracts). If player X is sold for £10 million but his valuation in the accounts is £5 million, then we have made a profit of £5 million - not £10 million. In the example that you use, I imagine that we actually made a rather big loss on the sale of Robbie Keane and only small profits on some of the others.
So this £27 million profit that you claim Spurs to have made on player trading is pure fiction.
* Amortisation can also be defined as the paying off of a debt on an intangible asset over a certain period.