Why do pigtails not count towards box volume capacity
Using an NEC Handbook, which is the NEC with more information and comments, I find that the pigtails are currently counted as part of the device fill rather than a separate conductor. All of this can be found in Article 314.16 and the associated tables.
The NEC Handbook has a set of separate commentary tables. In particular, Table 314.3 shows how different device allowances change based on the number of conductors are attached to it.