Could be because his technique is shit. It's likely easier to muscle .85 your 1rm 10 times than hitting one max rep requires better technique when you get to the 300s.
I just made this up but it sounds right.
Yeah, you made that up, but don't feel bad.
If you always lift with shit technique, then your 10 rep sets will be as shitty as your max attempts, and you'll still throw your back into them.
But, the 1rm calculator works the same regardless of your lifting style.
I used to do reps with horrible form and lots of cheating.
Now I'm old, and I just want to lift heavy without getting hurt.
The calculator worked when I was doing cheat reps, and it works now when I do proper form.
If you can do 10@225, then you can do 1@300, no matter how horrific the form ( assuming the form stays constant )
Edit: for the sticklers - if you do a perfect set of 10@225, you could probably cheat on a single and arch up 305, maybe 310 if you do a bridge, but that's not what we are talking about.