Excuse me for this, I put myself to use as Mentat. This post is long yet worthy with every portion of its parts essential.
Somewhere in part two of dune I believe I do remember a passage of the importance of water in the body while moving through the Erg, it's implied that one should drink often, but not deeply. That all moisture should be utilized.
It's also noted that the Fremen are perfectionists, it wouldn't be surprising if they would plan their own individual water allowance on a journey as per the capacity of the stillsuit and reclaim their own water back into their body cyclically throughout the excursion.
I imagine that a stillsuit is thought more as an extension, another organ, a second skin rather than explicitly as a tool to be used. Beyond the retrieval of water from the environment or held in literjons I think the water in the stillsuit would be too sacred to simply release into the reservoir, especially if the stillsuit is truly held in such reverence.
Beyond that no Fremen truly parts with the water of their body except for in death. No Fremen sacrifices their own water for those stupid enough to lose their own water. Water amongst the Fremen of the Sietch seems to only ever be shared from body to body during the tau orgy that follows the Bene Gesserit ritual of Agony or, of course, upon someone's death. Otherwise Fremen of the Sietch are only ever described either eating or with water claimed either through trade or in death, which goes to say either that particular Fremen is either quite crafty in mercantile or a bit murderous
On the whole reservoir idea, that water is being saved for terraforming, not personal use. That's why they still make good use of their own precipitation catchers, dew scrapers, and literjons.
On water balance itself. There's very little food in the desert and arid nature and white hot sun of Arrakis would likely make the water cycle much more intense, especially if we consider the fact that the night sky is reddish in hue, Arrakis is probably quite Massive if it red shifts the atmosphere. During the day Arrakis is described as having a grey haze of a sky, much like Mars does. If (assuming correlation with atmospheric density and light refraction and diffusement) knowing Mars has a thin atmosphere and also that Arrakis is massive it then the atmosphere of Arrakis is likely thin but relatively dense, dense enough to support life but thin enough to allow a lot of moisture loss by the white sun.
Also, again I can't cite the page but I remember reading that the atmosphere on Arrakis had an almost equal ratio of oxygen to hydrogen to nitrogen, with some other gases likely, maybe even spice itself. With a high amount of oxygen it would come as no surprise that evaporation occurs at a quicker rate, oxidation and what not.
So... It may just be the case that you really only lose just a thimble of water.
Beyond this, the body is in a constant state of chemical processing. Yeah, there's an extra cup of water if you do the math, but in a realistic practical setting you don't just have that sloshing around inside you unless you have to pee, but with the length of time it takes to digest food and the time frames excursions it seems unlikely that the half cup would be a present factor as the moisture in the food would still be going through electrolysis. That being said that water content mostly goes to surrounding tissues rather than going directly to the bladder or the skin, in an awful example this is why allied fighters in WW2 gave rescued peoples from the concentration camps food, however no water. Their bodies would've gone into toxic shock from the water so instead were given food to make up for lost moisture.
Done with my rant. It's still a thimble of water, and no one touches the Sietch reservoir.