It doesn't take much of a tilt, after all, the maximum tilt is 8 degrees on most TS lenses and typically only 1 or 2 degrees is used. While it is slightly different for a normal lens and tilting the whole camera, the fact that you are getting a significant amount of the ground level, suggests the sensor plane wasn't exactly perpendicular. Also, regardless of how good the resolution of a lens is, you can't overcome the laws of physics.