“The optical designs for these two lenses is completely different, the only things they share in common are 9 aperture blades, focal length, f-stop and both are produced by FUJINON in Japan.
The design brief for these 2 lenses was completely different. The XF23mm f2.0 was designed to deliver outstanding image quality without interfering with the optical finder of the X-Pro2. The X100T lens was designed to make the camera as small as possible, while delivering the highest image quality. 5 of the 8 lens elements of the X100T lens are within the cameras body and are of increasing diameter, this is possible because it is a fixed lens camera with a leaf shutter”
Sounds like a small difference, but whenever you try to miniaturize a lens you sacrifice some image quality.