No its not locked in- the amount you get ultimately (if you predict correctly) can be very different depending on what bets are placed after yours. Yes the actual percentage varies until the bet resolves. I think this is a problem they have (apart from not paying out larger bets!) in that there is no incentive to lace early bets- for example in the bet on whether BTC would reach USD$80k by 5th Nov you could wait until the last day when it was clear that it would not likely reach $USD80k and place your bet at the last moment.