Precisely the same happens to civvy project managers in DPA.
The mantra is "faster, cheaper, better" but you must always remember that your seniors, probably without consulting you, have told DEC "We can deliver to this time, this cost and this performance". If you better it, by definition you're contradicting the people who write your report. They don't like it and will mark you down in reports, advise you to seek another post and brief against you.
I'd like to say you're correct South Bound, but can quote many instances of the above over a long time - and very senior staffs (2* and above) have happily confirmed it in writing. Still got the letters.