For the NDB21 there's an obstacle of 570 amsl about 1.8 miles from the threshold just to the left of track. This, with 250 ft minimum obstacle clearance, dictates the OCA of 820 amsl for all approach speed categories.
For the ILS31 VM(C) minima, for different cats, different radii of turn are used to determine the area in which obstacles are relevant. For Cat A, the area does not include the 570 amsl obstacle, and so the VM(C) MDA can be 800 amsl. For cat B, the obstacle is within the area considered, and the MDA has to be 300 ft above the obstacle, presumably rounded to 900 amsl. For Cats C and D, it looks like the further 671 amsl obstacle is relevant, and with 400 ft obstacle clearance, the VM(C) MDA becomes 1100 amsl.