The reason is that circling minima can never be less than the straight-in minima. Consequently, differing minima for the runway approaches may result in the circling minima being raised for one runway only.
Circling minima are calculated independently of the navigation aid used, and have different obstacle clearance requirements and a fixed visibiity requirement.
Straight-in minima are determined dependent on the navigation aid used, and visibility requirements may vary dependent on visual aids available and the MDH. If these are greater than the equivalent circling minima, the circling values must be raised.