because the last thing they want to do is lose you and set themselves back in the hiring process. It’s a weird system where they formally offer you the position but the pay details are not complete.
The formal offer really is just opening up the negotiations; however, from the government’s perspective, they often overlook this and forget that these details are not agreed upon. And they are thinking, “Crap, we don’t want to set ourselves back. Give him something reasonable.”