track her soidum bro. she might be staying on her diet, but even diet coke and other zero calorie drink replacements have sodium in them which would cause this, esp since the fluctuation is very minimal.
i dont think its hormonal. hormonal wouldnt involve consistent daily fluctuations. sodium intake will.
get her drinking just water only for a week. zero condiments. you'll see it level out. then you can add them all back in cause you'll then know the cause, and how to control it.