LOL...That's exactly what happened when I used to use rain water years ago, the reading actually went down instead of up, but as time went on the reading would start to go up again. Useless to me as I had no idea of what I had produced back then.
This is apparently due to the organics and dissolved solids present in the rain water in combination with electrolysis. It was explained to me why the reading went down before it went up, but that was a long time ago and I would not be able to repeat that explanation even if I wanted to.
Of course it goes without saying that a reading is handy prior to brewing so one knows what water purity they had to begin with. That number is not so great as to cause any undue concern though - if using pure or distilled water that is - we are after all only talking about a 1-3ppm difference usually in the scheme of things, well that's all it is with the water I use here so it's not worth worrying myself over that.
The reading will still go down over the next few days anyway whilst some ions are forming more particles until the solution stabilises.
Similar to your 'diet'...LOL, nice and simple analogy, I like it. What's a few grams, or even kilo's in the scheme of things dieting? If one has lost 20 or 60 kilos or whatever, what's a few grams or the odd kilo here and there, something was observably achieved at the end of the day, and that's what counts .