Questions for rock-paper-scissors game

Kropiciel said:
I am watching Kaiji (at 7 ep atm) and some things are bothering me:
-Where did he get scissors card in the fight with the blonde guy? I mean, he was in possession of 10 rocks, 4 papers and 2 scissors till that moment, and after the fight they didn't fuss about having problem to draw. Later, we got an information that Kaiji had 5 scissors. So what, he bought 4 more? Why would he do that and why weren't we informed about it? Or were we?
-Why was there odd number of all cards on the floor for the most of the time? Cards were supposed to disappear in pairs. Okay, the 3 cards of the guy who tried to flush them in toilet, were taken out of the game, but as we were informed later, the board was left untouched, so there should always be an even sum. When Funai wanted to redistribute cards, the score board was showing for rock/paper/scissors 38/15/38 and then 38/15/37 for no reason. Why?
-They say, that if Kaiji had 30 rocks, 5 scissors and 34 papers, given the board was showing 38/15/37 respectively, he would have a chance to defeat scissors with 80% probability. Like seriously, where the hell did they get that number? If there were 15 - 5 (of Kaiji) - 3 (of flushed cards) = 7 scissors, 8 rocks and 3 papers on the floor, then, given a rock card, the probability of getting into a fight with scissors (=win) would equal 7/18 which is far far away from 80%. What's the author's math?