My guess (yes, guess....i've never sharpened a knife using a belt sander) is that the wear pattern is indicative of a mechanical inconsistency of the sander, like an "over-tightened" belt, as opposed to how the knife is run across the belt while the system is operational. Kindof like how when you "over-inflate" a tire, the wear is in the center...."under-inflate" and the wear pattern moves to the outer edges. However, it could also be that the rollers that the belt rides on could force the belts to not be even also...i dunno, haven't seen one <img src="/ubbthreads/images/graemlins/laugh.gif" alt="" />