I noticed that my squat strength exploded during the first couple months of the training protocol, and that was in part because–while I had a pretty strong bench and dead–my squat was super weak. I started with a training max of 220, and the next month I had to drop down to a max of 205.
My first week working off of 205, I only got 6 reps at 175. The next week I got 9 at 185. I don’t know what
happened, but the next month–with a training max of 215–I got 13 reps at 195 for my 3 day.
In my experience, and from what I’ve read around the interwebs, if your 5 day is around 10 reps, 3 around 6-8, and 1 around 3-5, then you’re doing pretty well, and there’s no real need to increase your training max, for as soon as you do, you plateau, take 90%, and before you know it, you’re working the exact same weights in a 2-cycle period.
Mark Rippetoe advises against using the type of max-based training because of the enormous gains that you can make when you’re a deconditioned athlete–or even just deconditioned in one lift. I was able to increase my training max from 215 in month 3 to 250 in month 4, and I was still getting 9 reps for 225 on my 3 day for squats.
If you are a deconditioned athlete making ridiculous gains in NERVOUS ability–your ability to fire more motor units at a time, dramatically improved technique, etc–then it may be okay to artificially inflate one of your maxes. To be fair, however, if you’re at the level where you can make such fast gains, 531 may not be for you. Rippetoe criticizes max-based programs for the deconditioned athlete for this very reason: why would you throttle your bench by adding 10 pounds to a training max in two months when you could add 30-40 in the same amount of time.
TL;DR: Like everyone else has said, the program is clearly working for you. Chances are you’re making insane gains because you were improving primarily through your nervous system. If you DO decide to increase the weight, be incredibly conservative in doing so.