It's called a PUNCHER'S chance and it's a boxing expression meaning the guy who has it is a puncher, which is distinguished from a boxer. Punchers, as compared to boxers, are less likely to win decisions as they tend to have lower output and land ratios. Usually, the boxer is expected to beat the puncher, but as you noted, this isn't always the case, so sometimes the "puncher's chance" is actually "more likely than not." But usually it comes down to how good the boxer is; if he's very good (like a Klitschko, for example) then the puncher's chance is probably pretty small unless he's a prime Tyson or some guy like that.
But a fight like Stout vs. Henderson is a fight where "puncher's chance" doesn't come into play at all, and I think you listing that shows your conception of "puncher's chance" was a little messed up. Neither of those guys are punchers. You need to have a puncher before you can talk about "puncher's chances."
And in MMA, obviously another big wrinkle is that a puncher can be taken down, so a worse puncher might have a better puncher's chance against a wrestler than a better puncher if he's better at keeping it standing. So the guys who actually had the best chance of KO'ing GSP weren't the punchers like Alves and Hardy, because they had no hope of staying upright and nothing for GSP off their backs to keep him cautious about trying takedowns. Condit had a better chance because his ground game allowed him to keep the fight standing and keep GSP's respect on the ground.
Obviously putting boxing terms into MMA is always a little messed up, but bottom line, you need somebody you can at least conceive of calling a puncher in there. A guy who gets 1 KO for every 5000 punches isn't a puncher.
