It's annoying, but there are soo many restrictions on federal plans I've given up on understanding them. We have GEHA (DH is a federal employee) and even when I had primary coverage through United, GEHA being my secondary through DH, every pharmacy in town claimed I couldn't pay the $4 "lesser" price even though my United copay was $10 because I had GEHA (federal) coverage as secondary.

So I constantly paid more for certain scripts, even though I had two forms of insurance, than people who walked in off the street without insurance did.
Luckily they only overlapped a few months and now I just get to deal with all the GEHA restrictions.