Interesting case, that "if not". I think it can both negate what follows, or confirm it. If we remove "exactly" from two of the examples quoted by JPablo above, I find them all more or less ambiguous. (Or is it just me?)
Here's what the Oxford Spanish Dictionary says:
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They were undernourished, if not (yet) actually starving: estaban desnutridos, si bien no se estaban muriendo de inanición.
She was very offhand, if not downright rude: estuvo muy brusca, por no decir verdaderamente grosera.
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Notice the "(yet)" to make the meaning clear, and that both the examples are with "weaker/stronger pairs".
When the pairs are unrelated, I find it more natural to interpret what follows "if not" as a negation, though, as irmamar's book did.
Saludos
