I haven't used mine (the 9.5 version) enough to verify this, but supposedly the difference between the two lenses is that the 9.5 was designed for banquet cameras, has minimal movements, but achieves maximum sharpness at nearly wide open. This makes sense for a lens designed to be used for group shots inside with relatively darker light, achieving front-back focus by simply tilting the back, with no need to engage in perspective correction. The 12.5 version is more normal, for regular view camera use, has wider coverage, allows more movements, and needs to be stopped down for best quality and coverage.

For future reference, if a normally good lens is sharp in the middle and really bad as it moves from the center, the very first thing to do is check for flipped element/s.