The official standing is 1,084 which is probably at 4/sqm ( equivalent of seating +standing of 145 per car and given 190+/car is just possible on some existing stock so at 7/sqm it would be nuts (over 2500 for a 12 car)
I'm by no means an expert - but I wouldn't be surprised if they took something crazy like that as the crush loading conditions. It's a common occurrence in engineering to go a little over-the-top in extreme cases to give it a bit of built-in redundancy.
If they planned for 4/sqm, and that occurred throughout the train but one more person squeezed on, you'd be in a situation where everybody could feasibly fit in but the train is now heavier than it's been safely designed/tested for. If you sort it out at 7/sqm you're confident that it's never actually going to go above that, and you're in the clear.
Much safer in engineering to over-compensate, than to build it right "on the money" and risk being prone to incredibly extreme conditions.