AGP can operate at either 3.3v (official) or 1.5v (official). Now, if you take 3.45v which Asus often uses by default for the 3.3v line, and divide by 3.3, then multiply by 1.5, you get 1.57v. That's close enough to 1.58v in my book.
You see, 1.5v is taken off the 3.3v line.
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