What was my mistake? I planed the sideboards perfectly even with each other. However, during assembly I noticed a defect and flipped one of them end for end so the other side would be out. The sideboards were 1/16th narrower on one end so this introduced 1/8" of wind. A foolish mistake, but one I could recover from.
Here is a picture from the bottom. I glued the sideboard to the top board on each edge and then used screws with washers to attach the top boards to the cleats. The washers make it easy for the top to slide back and forth in the elongated holes as the moisture content changes seasonally. This will always be my favorite photo of the bench because it really shows how smart the Nicholson bench design is. Siimple, straightforward, highly functional, it hides its sophistication.