Eliminating potential meanings of an ambiguous phrase because if they meant XYZ, they'd have written differently: What is this inference called?

Question

I find this situation arising from time to time:

1. There's an ambiguous phrase Z with multiple possible meanings, X and Y (for illustration).

2. If the author meant X, a reasonable or necessary choice of phrasing is Z.

3. If the author meant Y, they could (and hence would) have written the phrase differently in order to avoid ambiguity.

4. Therefore I conclude that the author does not mean Y, and hence means X.

I find it particularly important to be aware of this in technical writing: you need to write in the expected way.

I'm just after a name for this process...

Question: What's this inference process called?

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Example 1:

There is a 12-page limit which could include references.

I infer that the page limit does not include references (or at least, it doesn't always include references), since if it did, they would have simply written:

There is a 12-page limit which includes references.

Example 2:

This also arose recently in my Chinese.SE question with the example:

我有两个姐妹
I have two sisters

Without further inference, the above could mean older and/or younger sisters. However, if I meant anything other than "one older and one younger", I would have said either:

我有两个姐姐
I have two older sisters

我有两个妹妹
I have two younger sisters

Thus we infer I have one older and one younger sister.

Answer 1

I'd say "implicature" or "using Grice's Maxims" (specifically, the maxim of quantity, I think)