Since I'm on AFPS75 this didn't apply to me so I sort of switched off to this bit but I seem to recall that if they give you more than the proposed amount then HMRC would view it as a taxable benefit and not only would that then reduce the amount you'd get by the tax amount but would also count against you in terms of the total amount the banks would lend to you because it would be counted as a future debt. (or words to that effect)
Feel free to correct my vague recollection of events.