Hi Nick
The higher limit is based on an obstacle in the missed approach procedure and because at the standard assumed climb gradient that obstacle will not be cleared by the required minima the minimum descent altitude has to be raised from the minima calculated for the final approach sector on its own. However, if an aircraft can climb at 4.3% the the obstacle can be cleared from the lower minimum descent altitude. An MDA(H) has to be published based on the standard climb gradient.
Cheers
TeeS