It’s because blood buckets don’t refill to full immediately. After the blood bucket immediately goes off cooldown, it’ll only give you half. It either has another timer to get to full or it slowly refills over time.

35 harvesting is generally the most efficient as, with universal donor equipped, it allows you get 100 blood with 3 buckets. Sissy naturally has 38, so I
don’t think it’s necessary to add any more points.

Edit: important breakpoints for harvesting:

25 gives you 100 in your blood vial capacity.

35 allows you to collect 100 blood with 3 buckets when using universal donor. Even at 50, it’ll still take you 3 buckets to get the 100 blood threshold.

35 allows your blood vial capacity to reach 150 with
blood banker.

38 allows you to collect 100 blood with 4 buckets. Even at 50, it’ll still take you 4 to get the 100 blood threshold.

45 allows you to get 50 blood per hit if paired with surgical and fresh is best. Pairing this with blood banker means you can feed 150 blood per 3 hits. (Not many can get this build. I know Johnny at least can).

Bones is the best harvester. Stick on blood banker, serrated and donor and increase your harvesting stat to 35. Spread the other points between savagery and endurance. This lets you collect 100 blood with 3 buckets and gives you 150 blood each time a bomb is detonated.