Any ideas why they're different? I was under the impression the circling area considered would be the same no matter what IAP you execute to get to it, hence all the obstacles are common no?
The obstacles which affect the approach minimums aren't just what's around the airport in the protected circling area...you've got to consider the approach path. Minimums are determined based on obstacle clearance criteria, as well as what's called an "obstacle clearance plane," or an imaginary surface or plane through which no obstacles can penetrate. If obstacles do penetrate this plane, then it's angle must be raised, or floors raised for a particular segment of the procedure. The reasons for the slightly higher minimums may have nothing to do with actually circling...because you'll reach the MDA for that procedure well before you actually start circling. It may have to do with obstacles farther out between the last step-down fix or FAF and the missed approach point...which don't affect the other procedure. It's very possible to having circling approaches to a runway complex with different approach minimums...because obstacle protection while actually circling is only part of the picture.
Think of it this way: by the time you get to the circling part of the approach, you've already flown the procedure...and the minimums apply to the procedure itself...The minimums are established where they are for a good reason. We may not know what it is, and it may have nothing to do with the circling areas...but may well have to do with obstacles, airspace, or other considerations that were a factor during the final approach.