Different clays behave differently, so a mixed box is not simply the same bowl in three colours. The pale bodies are finer and take a crisper edge, the red earthenware is coarser and more forgiving, and each has to be formed and dried at its own pace before anything goes in the kiln.
The glazed pieces are fired twice. Once to harden the clay, then glazed and fired again, which means they spend longer in production and shrink further than the unglazed ones. Sockets are cut with that second shrink already allowed for.
Boxes are made up at the end from what a firing produced rather than to a fixed ratio. If a shop needs a particular balance of glazed to unglazed, say so on the inquiry and we will put the box together that way.