John has claimed a billion times he is positive he will win, and his reason for asking to bet 20k was to maximize how much he was going to win.
So think this out as if it were you. There is a guy at the pool hall you are positive you can beat, but you two have never managed to make a game in all the years the both of you have barked at each other back and forth. Finally one day when you bark at him to play for x amount on such and such date a few months down the road, and he agrees to it.
But after thinking about it, you decide you want to play for an amount that is twice as much (2x) because you know you are only going to get to play this guy once in your life since you dislike each other, and you want to make it count and win as much as you possibly can off of him the one and only time you are going to ever play.
So the next time you see him you tell him you want to make the bet for 2x, twice the amount you have previously agreed on. He doesn't give you an answer to your offer to play for 2x. So every time you see him for the next few months leading up to your match date, you bark and bark, and beg and plead, and prob and probe, and insult and push and try every which way you can to get him to play for 2x, but he more or less just ignores you and you never get an answer.
Finally after years of barking at him, and after months of pushing for him to up the bet to 2x every time you see him so you can win more money off him in your one and only match, he finally answers you and says "sure, I'll play for 2x."
You mean to tell me you would really say well let me think about it some, I'll still play for x, but for 2x I will have to thing about it. You are full of dump truck loads of bullsh!t if you try to claim that you, or anyone else in the world would really do this.
What you would do is snap jump on that offer and accept it so fast it made his head spin. You wouldn't want to risk him changing his mind after all that. Use some logic dude.
The only reason someone wouldn't snap call it after all that is if they weren't really trying to up the bet, but just trying to get in their opponents head. And when the opponent agreed to the 2x, now you have to think about it to see what you really want to do because you never were very confident about your chances, so you want to wait until match day to see how confident you feel that day, see how well you are hitting the balls that day, see how well your opponent is playing when he warms up and how confident he looks, and then ultimately make a decision about whether you really want to bet more or not.