The BLS study is used to compute base pay scales, hourly and salaried, and does not included benefits. The Heritage study specifically includes benefits payed while employed and post-retirement.
As Mookie stated, different data sets.
I grew up in a family, both close and extended, of civil servants. DoD specifically. My brother just retired from the DoD. Believe me, the benefits are second to none for rank and file employees.
I wish I could offer my people anything close to what the feds give.
Also, I'm not surprised a federal bureau came up with its own study showing they are not payed enough.