Interesting! For what it's worth, I also learnt that there are quite a few lens manufacturers out there and they all differ slightly regarding lens thickness and refraction properties. That may be another explanation.

And, if your optician is close by and shares your belief that your values vary over time, getting your values checked in different "states" (morning / evening / tired / etc.) can be helpful for deciding on what the "right" values for your glasses may be.