I guess men vs. women is a rough guide to body size. Using weight might have been more accurate than setting a higher interval for women. Donor centers want to err on the side of safety and they don't seem to care about optimizing it precisely.
I'm quite sure the longer interval for women is not because they generally weigh less than men, but because they also lose blood during their periods. Or it might be both. But usually it takes a little longer for women to get their haemoglobin level back up.
Recovering from donating blood is very individual, and factors in determining it are weight, diet, gender, age, exercise level and genetics, and so it would be very difficult to optimize it more precisely. The interval is a bit on the safe side, agreed, but I've had times when my haemoglobin levels didn't come back up to a normal level in 60 days, so I took a bit longer break and changed my diet so that I got more iron from food.
I'm glad the U.S. interval is 56 days instead of 60 days so we can make a habit of going on the same day of the week each time.
Yeah, it's not a big difference, but makes determining the next day a lot easier. The Finnish Red Cross Blood Service has a counter on their website, though, with which you can check your next available donation date, it'll even email you as a reminder.
It's the same for us here, except that Americans don't understand the metric system (it's too complicated with all those powers of ten

) so we have to round it up to a pint!
Heh, yeah, it's really difficult.

So it's a US pint, 473 ml, not Imperial, which is 568 ml. Just checking.
This is OT, but it's funny that here we've used to SI measures (metric system), but now some beer companies have come up with a Imperial pint sized beer can (=5,68 dl). It's usually a whole lot more expensive than a half litre one, even though it has only 13,6% more content. Another "fun" thing is that it's quite difficult for people to let go of talking about engine power in horsepower, even though it should be measured in kilowatts. It can be difficult with hp(E), hp(S), and PS...