The dealer you purchased the lens from ordered it that way (a dealer option). Unless Zeiss changed the specs (a possibility), the standard lens flange on the 25 ZM brings up 28mm framelines. I have first-hand experience with this, owning two examples of this lens - both were 'unmodified' out of the box bringing up 28mm framelines. In each case I had to order a new lens flange so the lens could be coded as a 24mm Asph. Elmarit.
Another recent change from Zeiss is they have shifted the mount so 6-bit coding can be done unhindered by a screw (it's about time).