Halo 2 Anniversary Terminal Achievements...

Two days ago when I was playing H2A (Master Chief Collection) I had set out to find and unlock all of the achievements for the terminals that are hidden throughout all four (not counting Halo 3 ODST) games. There were two or three terminals that did not have achievements on them, but later I finally recieved an achievement for a terminal that was on the mission I was playing at the time. Is this normal? Do some terminals not come with achievements?