From the way I understand it, it is a percentage of the unit. Not all units are the same size so that may be why it is different.
Points are simply a way of represent that. For example, if a unit comprised of one grad villa for 50,000 points, and someone buys 250, it would represent .005% of that unit.
But, if someone buys that same 250 but is deeded to a unit that includes 300,000 points, then their share would be .0008%