Section 3(1) of Evid Act: Double Meaning or Ambiguity?

I'm genuinely stumped by this - Section 3(1) talks about exclusion of oral testimony on a document due to ambiguity or uncertainty in the language used. But then it says if there's double meaning, the meaning which goes against the person who is interested can be adopted. But how is this different from a contradictory interpretation? Isn't double meaning and ambiguity basically the same thing? How can you adopt a meaning that's against the person interested without making it ambiguous? I'm totally lost here.