"Hmm... these Terminators have a 2+ save. Why?"
"'Cuz they are wearing the greatest of the ancient armors, maintained through the arcane rituals of the Tech-Priests. Of these rituals... goes on for a while..."
"Yeah, that's nice. But they don't provide any protection against meltas?"
"That's <Purity Seal>!"
"Hmm... let's give 'em a 5+ Invulnerable save. Now, we'll write up a nice little fiction piece and nobody will be the wiser..."
Three months later...
"Why the <Purity Seal> does the Terminator get a save against Mind War??? See, Mind War is a mental attack, used by the greatest of psykers among the Eldar, and everyone knows that the Eldar...goes on for another while..."