>The only way to understand the intent of the written words is to ask the original author.
But likely the original author of the spell is not the same person as the original author of the background option, which means some collisions are going to happen. The spell, which has worked that way since the original Spell Law, is obviously modeled on the D&D version; it's an unusual mechanic that occurs in few places (Repel Undead is vaguely similar, but not quite). The background option is written based on how spells normally work.