But if a booking change requires a ticket re-issue (which these days is most changes), then AIUI the TFC are re-priced on the day of re-issue. Where there has been some exchange rate movement since the original ticket issue, the sterling equivalent of some of the TFC elements that are priced in a foreign currency may change. IME this is a common cause of small changes in TFC on re-issue. And because it's exchange rates, sometimes this is in your favour and you get a small refund.