Mine does move around a bit from day to day without really obvious cause. Once every few weeks I keep a really, really detailed food diary (it takes too much effort to do all the time, for me anyway). Every single thing that goes in my mouth is written down. This shows me that there is more variation between days than I thought, a gingernut here, a few almonds there, a different brand or type of yoghurt, etc.
Might be worth trying if you're not doing it already.