This is a real headscratcher. Randomly while in Blender my snapping will change from meters (I.E.) 6m to 7m, to smaller increments... 6.17m, 6.25m, and back again with no indication.
I thought it might be object to object, or alt + tabbing (to follow along) but it's making the exercise very difficult... lining things up somewhere between 0.399m and 0.401m
It seems to happen when hitting shift to change to smaller increments, and sticks around
It seems that canceling certain actions fixes this behavior. Right-clicking to cancel movement after adding an edge ring or an extrude for example, then (G)rab and X to move in increments along the x... doesn't completely solve the problem, but I'm able to progress
Edit: This is only part of the solution. The other part is related to zoom level.
In that case, it sounds like a bug to me. Please report it if it's reproducible! https://developer.blender.org/
The answer was zoom level. At different zoom levels the increments change values. Not sure how to disable this.