The back of the watch is very much curved. It's not as noticeable when people are wearing them because the watch sits right in between your wrist bones. https://i.gzn.jp/img/2018/09/21/apple-watch-series-4-open/p4...

It's flat enough to stay inside a 7mm charging range. (Or the inch plus that newer chargers can supposedly do.)

gah. You're right, I'm wrong. Dumb.