Rounding Error.

Hey demonos, could it be that some of the gear you change has small amounts of energy shield? Or even flat regen? Else if you have any source of energy shield like rise of the phoenix, changing your life total by a lot will definitely make a difference because the proportion of life-energy shield is gonna change significantly.

Though I can totally see the truncation of a few decimals causing the difference between sustain and non-sustained rf when you're super borderline especially you are a non rise of the phoenix build RF (maybe you use a -%degen taken leo shield)

The game should first sum all sources of reduced mana reserved:
8% + 2% = 10%

Then multiply each item that reserves mana by
(100% - total % reduced mana reserved) without rounding:
50 * 0.9 = 45%
35 * 0.9 = 31.5%
25 * 0.9 = 22.5%

45% + 31.5% + 22.5% = 99%