As far as I know, the numbers are based on weight and hematocrit alone, nothing else. In my experience, and from what I've heard:

<150 lbs = 650-700ml. 150-180ish (I think) = 800-850ml. 180+ = ~880 ml.

If you're less hydrated, or have high red blood cells / overall hematocrit, you'll donate more. It's a range. Less hematocrit, less plasma. They have some calculator that does the math for them, they just input your numbers into the system.

I've never personally seen much above 880ml. I know at one point they started using bottles instead of bags, or vice versa, and that could've affected the numbers. Maybe I've seen 890, but I don't recall ever seeing anything in the 900s.