Not uncommon even with expensive models.

I have heard several theories about why that happens. Temperature differentials from the probe and solution, even on ATC models, causing it to constantly readjust as everything reaches equilibrium. Weakly buffered solutions not conducting a current very well or consistently. And even that the current can change the pH on the surface of the bulb slightly so moving it around in the solution helps equalize it.

Then there is always that a lot pH pens are just pieces of **** and poorly made.

Seems that moving the probe around fixes whatever issue there was previously. I'm now getting essentially rock-solid readings. Thanks so much!