You can do 405x12, but you can't pull 500 once
...I train HIT-style so I can manage quite a few reps with weights close on my 1RM.
For example, six months ago I was deadlifting 405 for 5 or 6 good clean reps... but an attempt at 455 proved I couldn't get it past my knees.
When you train HIT you don't exhaust your stabilizer muscles and can do work sets very close to your 1RMs, closer than you could possibly get doing conventional training. A good example that everyone can relate to would be the bench press. I can bench 315 max (I'm only 5'5''), but I can do a work set with 245 for 11 reps.
I'm a little shy of attempting a full 500 this side of a 405x15 work set. Any powerlifters who want to comment on this logic please feel free to do so as I'd like a powerlifter perspective on my reasoning.
The Luke