This is speculation without seeing the actual design info, but ....
There are generally three sets of icing shapes considered in certifying an aircraft - the fully developed ice shapes (which exist only on unprotected parts of the airframe), the "failure" ice shapes, representing the worst case ice accumulation on the protected parts of the airframe following a failure, and the "encounter" or "delayed turn on" (DTO) shapes, representing the ice that will accumulated on an unexpected encounter with ice, before the system is turned on.
Since the conditions being discussed are not failure cases of the icing system, we only need be concerned with the unprotected ice (often called 3 inch ice, since that's the standard height assumed by most) and the DTO ice.
3 inch ice will be on all the unprotected surfaces when in icing conditions; DTO has to be assumed even for "no ice" conditions.
IF the location of the unprotected area is such that it affects the stall onset only at flaps 15 then that would explain the reason for an adder only for that case. It is possible that it actually affects all the configurations relative to a clean wing, but that landing flaps are sufficiently affected by DTO - which is considered for all conditions - that the 3 inch effect is masked.
Since flaps can cause the stall to initiate at different spanwise locations, this is a possible explanation.