There’s an interesting point about any flat disc made on a lathe. Chances are one side will be ”convex” and the other ”concave”. The reason is how they are parted off by the lathe. The parting tool is adjusted to make both sides flat (neither concave nor convex) BUT just in how the wood is cut, there will most likely be concave/convex sides.
In other words, we don’t ask for our discs to be concave/convex but they are ever so slightly this way. It’s just by chance that they are. We cannot guarantee to what degree.