I interpret the "19% spend more than £100 each time they play" stat to mean ‘stake’ in place in ‘spend’.
That really isn’t surprising on the roulette, blackjack and fruit machines. The actual losses of a punter in sessions with £100+ staked are probably a small percentage of that total. I am an affiliate for a casino site and have a player who regularly stakes more than $1000 per day at online slots games. The total profit from this player is usually only about $40 per day.