Why does the contraction "she's" mean she is or she has?

Question

I saw this from globalnews.ca:

Molly Johnson on the album she’s always wanted to make

When referring to google ngram, I get 3 possible combinations of she's:

She 's
She's
She has

So my question is should she has be contracted as she 's in the above example like in the examples found from google ngram to avoid confusion? Google ngram hasn't been exactly consistent about this, sometimes using she 's to refer to she is and she has.

Coming across the usage such as that from globalnews.ca does not look right to me and it wasn't until today I found out the she's means both she is and she has-- modern usages follow this pattern too. So as grammarians do you think the contracted form of she has should be she 's?

More importantly, are there rules for contracting words? Say, if I wanted to express She was as a contraction could it also be she 's or she's:

She's* tired

*Although is and was are both be verbs, both have tenses relating to different time periods - the present and past.

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EDIT:

I asked this question a while ago and most people’s answers or comments tend to be that there is no rule regarding this usage of the contraction “she’s” or contractions at all, but doing some digging (by accident) I cane upon this answer regarding clitics:

The clitic 's meaning "is" can only be used to substitute for a "weak form" is (pronounced /əz/). The is in in "Where is it" is the "strong form" is (pronounced /ɪz/) since it is used as a main verb and not as a modal (or helping) verb. Therefore it cannot be replaced with the clitic 's.

See a related answer I wrote a while back discussing this restriction in my answer to Is there some rule against ending a sentence with the contraction “it’s” - answer by nohat

Source: Why is 'Where's it' Grammatically incorrect?

Could someone entertain me on this subject of clitics and apply whatever rule CaGEL has to some of my examples in this question? As I simply do not understand it.

Answer 1

Simple answer: because that's the way English works.

I am     -> I'm      ||  I have     -> I've
You are  -> You're   ||  You have   -> You've
He is    -> He's     ||  He has     -> He's
She is   -> She's    ||  She has    -> She's
It is    -> It's     ||  It has     -> It's
We are   -> We're    ||  We have    -> We've
They are -> They're  ||  They have  -> They've

There is a certain amount of ambiguitity here, but the precise meaning can usually be determined by context. You'll notice that the same ambiguity exists with he and it is/has.

Furthermore, it's (it is / it has) can also be confused with the possessive its (note no apostrophe)!

There are no contractions (that I'm aware of) for was (the past tense). She was is always She was, but She had can be contracted to She'd.