I agree with this 100% ^^^^
I had a WFT trimmer and would chase the coal all over, adjusting back and forth. I got a Frankfort Arsenal trimmer which bases off the shoulder as well. Same result, but I paid extra attention to the brass. The brass would come out consistent, depending on the headstamps (mixed brass - various collected) and I came to the conclusion it was rifle chamber differences. Since I began using more of my own brass (fired in my own gun) versus a hodgepodge, the consistency was more obvious. If you look at the shoulders and neck of 223/556 brass, you can see some extremes of well-defined sharp lines/angles and some brass just seems rounded at the shoulder edges.