A porcelain mug has pulled slightly oval because of the weight of the handle. This happens in highly vitrified porcelains (e.g. translucent ones). The amount of feldspar or frit in the body determines the degree of maturity, the correct percentage is a balance between enough to get the maximum translucency and hardness but not so much that ware is deforming excessively during firing. This is Plainsman Polar Ice at cone 6, this degree of warp is acceptable and can be compensated for.
A term used in the ceramics industry to signify the degree of vitrification in a fired clay. Mature clays are dense and strong, immature ones porous and weak.
Warping happens during the firing of ceramic ware when there is a high degree of vitrification or a shape is unstable. But warping is expected in translucent ware, it is just a factor that must be compensated for.