There are salary tiers that are the same across all government employees. A new grad software engineer with a master's degree will earn the same as a new grad biologist, even though the outside government salaries for the two might differ by a factor of 2. There is no practical way of paying somebody more than this narrow wage band even if it's impossible to fill the position with somebody competent. You end up not having good engineers working for the government, unless they are willing to take an extremely steep pay cut.
>>jupp0r+(OP)
OK, I'm not sure I would describe that phenomena as an "equality principle" though. That is just a misleading label for a misguided bureaucratic rule.