Hehe you make sense! I think we have all gone (or in my case still are going) through it.
Two things I can think of are 1. Try levelling with a semi circular object (see Scottie Callaghans barista tools, aka grinder lid or yoghurt lid or tuppaware lid that your wife mysteriously cant find anymore). Vary the diameter up to flat until you find the one that works. 2. How about weighing the whole pf? Get some accurate scales and tare them with an empty pf.
And just thought of 3. Use your grinder timer. The MM is a little hard to set but should give reasonably consistent results if you dont change the grind.
Which brings me to another point in my wandering brain! Are you sure the changes in volume arent related to old grinds in the doser or some other technique issue? I find that not paying attention to my Giotto will only change a 25s pour by <5s either way...