To a comment in Picture This CX, I replied that warped Windsor seats were not uncommon.
A few minutes flicking through the archives returned the following additional examples of warpiness. (That is a word. Now.)
Common Elm is particularly prone to warping, however as a great number of these chairs were intended for outdoor use, it’s not inconceivable that some were left out in the rain on occasion, the effect of which could well be a warped seat no matter what the variety of elm.