As far as I know, "if not" normally means something in the vein of "perhaps even", such as in the example phrase "We might be able to hunt down three, if not four rabbits with the ammo we have".
But I've encountered an example of usage where I'm not sure ifthe user's intentions reflect the phrase's intended meaning. The excerpt is:
"A newly initiated practitioner of the dark arts, Arcanus is taking his first steps down the path which he hopes will lead him to power and glory. Strong in mind if not body, he has concentrated his time in pursuit of magical knowledge."
Necromancers in that particular game (it's Might & Magic 8 btw) are the frailest and physically weakest class by design, as is generally common with spellcaster classes in RPG games. The usage of "if not" in this case therefore wouldn't be justified in having vibes of "strong in mind, and who knows, maybe even body?" I would be sooner inclined to understand it as another way of saying "so strong in mind that it's no harm done that he's weak in body".
What are your impressions?