Pfft, that happens all the time. The bit of the story (maybe) you are missing is where the guy does 10kg less and lifts it like he's got 40kg in the bank (I speak from experience of doing exactly the same). It's hard to describe how things can suddenly go to shit (for some people). There's nothing wrong with that lift, at the top end of your deadlift, sometimes you know it's just not right to even start. The dude did the right thing. The worst thing you can do is try to grind through a deadlift that you know is pretty much a cointoss on whether you make it or you back disintegrates. I know because I have many times and the good portion of the next year I spent not deadlifting, due to the injury I gave myself when I finally got owned by it - definitely wasn't worth it. Once you have stuffed your back from that, if you're anything like me, if that bar doesn't have good acceleration off the bottom, you just don't do it (hence, 10kg less can go up like a rocket and bar can not move with more weight). Eat humble pie this week, get to deadlift next week.
He probably needs to do deficits.
