A calibrated set of humbuckers(or single coils for that matter) are pickups wound to optimise tone and output for their respective position, ie bridge and neck humbuckers are wound differently and often feature different magnets too. The strings move alot more over the neck pickup and generate more current so neck pickups are often wound to have a lower output in comparison to a bridge in order to achieve better clarity and balance between the pair. The best way to think of a calibrated set is as a matched pair.
