Yeah I know what you mean mate.. reminds me of people who say they can squat huge amounts of weight then when you you see them they're doing quarter squats.... they're not training their bodies, they're training their egos. As you say boxing has alot of those sort.
I peaked at 25 a few years back. I was fit and strong and training them almost everyday. These days I'd be lucky to get out 8 with good form. Once I get my garage sorted out as a gym I'll start training the movement properly again.
I'm willing to bet that 90% of people here do quarter range motion kipping style pull ups without bringing their chin above the bar.
It might sound strange but I've experienced the same issue where I was doing 15 chins but could only manage 3-4 pull ups (and not very strict ones at that). One reason might be that chin ups recruit more muscles = greater assistance, spread of load, larger muscle (lats).