All the fret markers (dots) in this photo should be centered between the lines on either side.
When I make fretboards, I make those holes, fill them with birch dowels and super glue, run a parallel pass to radius the fretboard, and then cut the fret slots.
Judging by the radius, it seems like the toolpaths were off before the fret slots were cut.
I did it again this morning and slowed my feed down to 50 in/min and it seemed to be off in the same exact direction (Y) by the same amount.
I often use the center as my origin. I draw lines from the 4 corners to locate the center. Before cutting use the jog menu and the rapid position for X and Y and the Z for +6MM to make sure you are where you want to be. I always put a vee bit in to locate the exact center because larger round cutters can trick you to not be centered.
I have an L bracket I use to help square a project on the spoilboard. I place the L bracket down and use the jog function to make sure it is square to the router. Then I tighten down the L bracket and recheck to make sure it did not move during tightening. If you project is not square you can have issues with it cutting off center. Plus using the center as origin if you slightly miscalculated the dimensions of the stock the cut will be centered on the project.
I use the painters tape and superglue a lot on small projects. I use the L bracket to place the object and if it will cut close to the edge I remove the L bracket. Before removing the project I put the L bracket back in place so it stays square to the router. When I created my spoilboard I made sure to adjust it to be square with the router. I have an SO3 with a Tee nut 2" spacing. The newer machines have the hybrid tables and they should be square with the router but you need to verify it is.
Since you mentioned belts, I am guessing a shapeoko 3 if that is the case also check the eccentric nuts and the v-wheels. I have noticed that has caused me to miss steps as well as the belt tension.
Funny how my electric glass top stove looks like his. I try to keep it clean but it is almost impossible. Everything shows on a black glass stove top. It is amazing how every little drop that gets spilled gets burned on. Just stirring something you get very small drops splash out and gets embedded on the stove top.
Things happen. A long time ago when I was in the USAF I was transferring from Alaska to Austin Texas. My wife came back before I did and I ordered a pizza and had some left over. So the next day I turned on the oven to warm it up. About 3 days later I realized the stove was still on with the crispy pizza in side. So ever since when I turn on the oven I always open the door to see if there is anything inside.
Some habits are hard to break while others are making you safer.
