I did a few further tests with a steel ruler. With that, only one of the sides has discernible cupping, and I'd say it's neglible. I then checked the back of the razor with the steel ruler and indeed it seems to be slightly uneven. Not much but enough to be visible against the light. I also checked the stone I have been using but it seems completely flat. (I don't know if what I've got is actually a steel ruler - it's one of those used to ensure 90 degree angles for carpenters etc. It is mad of steel though. I wouldn't put my head on the block that it's made to a greater accuracy than the razor.)

I'd be glad to take a look at it if you like.
Thanks, but I'm beginning to think it may be my technique that is to blame more than any irregularities in the paddle. I agree entirely that it ought not cause irregular wear when using the x pattern.

I might try to get a picture of the paddle and steel ruler held to the light later today to get your opinion.