This!
In particular, this -I struggled with this issue on and off for literal years. I spent untold hours and good money trying to sort out exactly the issue you are describing. I chewed through an entire LN blade trying to figure out what was causing "uneven" honing of *some* of my blades. I finally traced it back to haphazard application of the ruler trick, which was creating an inconsistent bevel on the back of my irons. I'm not against the ruler trick, and I continue to use it, but it absolutely has a dark side. This dark side seems to be relatively uncommon, as posts describing the issue you're describing are quite rare (i've seen them all), but when the issue pops up, it's a bear to diagnose.