He put the ultra thin strip between the cone and the headstock of the lathe. When he started to recut the drum, it cut noticeably crooked. So, that little bit can throw it off and the only way to get it right is to make sure every part is clean, and it's on straight to start with. And if the tool bits are not sharp or the feed is set too high or - - - -. But bell mouthed or out of round should not be setup problems. Out of round could be uneven cutting due to hard spots or the drums just got hot from use which can happen real fast.