Re-reading the rules, I'd say that Cold-Dragon is right about Invisibility. Being an indirect illusion, (Improved) Invisibility does not actually target anything, thus the static "Target" number. In fact, Indirect Illusions work by actually "creating" the image (or, in this case, the non-image) for people to see. This differs from a Directed Illusion like, say, Chaotic World, which is actually cast on a target (or several targets). Both are resisted because they are magical effects, but the OR doesn't apply to Indirect Illusions because you're not actually doing anything to it. If this weren't so then there would be no mechanical difference at all between Direct and Indirect Illusions.
The real problem here is that SR is very inconsistent about the wording it uses. "Affect", "Target", "Subject," and "cast on" are all used interchangably in the actual rules and the flavor text, which makes it hard to dicipher what each actually means.
(Edit): Oi. Maybe I'm just tired today. I'm sure I've seen this explained before. Ah well. nevermind. I concede.