I got an Oyster card last October and I noticed there was a deposit for the card. Mine was 5 pounds. My sister had got a card the previous year or even in 2010, and her deposit was 3 pounds.
Imagine this arbitrage opportunity:
Get 100 Oyster before year end/February/whenever they rise the deposit. Assume they raise it by one pound. Return the Oysters and get the new deposit for a gain of 100 pounds.