The drain pipe is as close to the center as possible. When you tilt a cylinder half full of water, the point at the center remains at the same height. Consideri

The drain pipe is as close to the center as possible. When you tilt a cylinder half full of water, the point at the center remains at the same height. Considering the cups are slightly wider at the top than the bottom, it can hold more liquid when tilted, so the height at the center would be a bit lower.