Well, there is no easily guessable logic in their system. This probably is intentional, like in most gambling systems.

Assume (hypothetically) that main business process of OT is borrowing interest-free money from gamblers (OK, would-be upgraders, but it doesn't matter). In that case selling upgrades as much as possible would NOT be among their main business targets. Instead, maintaining cash flow will be. All other parameters would be among business controls to keep this system going. They need to really upgrade some (adjustable) part of requests, just to keep OT offers still interesting enough for passengers. They need to adjust also upgrade prices for passengers, and amount of money they pay to airlines, to keep all this interesting also for airlines. All-in-all this seems to be very interesting kind of gambling business which needs very careful adjusting all the time but probably keeps some considerable amount of money available interest-free. Real profit might come from using this money in financial markets, that is, from another kind of gambling

That's fine with me as far as I am still getting upgraded sometimes