A postcard from my friend Chris Ambidge, an ad for the comedy movie Stiff Luv (2008), picturing six cast members, all dressed as women:
Something tells me, Arnold, that not all these women actually are.
(To judge from the cast list at the movie's website, I'd guess that NONE of the women actually are.)
Ok, Chris's note is a joke. The sentence isn't grammatical (though it's entirely comprehensible). But what's wrong with it?
It's our old friend Verb Phrase Ellipsis (VPE), though at first glance that might not be obvious, because the elliptical constituent is a NP rather than a VP. But as I've pointed out in the past, VPE is just a label, not a definition; most instances of the construction are missing a VP, but some are missing other types of phrases. What's crucial in VPE is an elliptical complement of an auxiliary verb, with the ellipsis anaphoric to a constituent in the linguistic context. A straightforward example (with the antecedent bold-faced and the position of the ellipsis indicated by an underscore):
A: You can't do this. B: Yes, I can ___ ['do this'].
Now a (grammatical) example with an elliptical predicative (a complement of a form of the auxiliary verb BE), which could be AdjP, PP, or, as in this case, NP):
Not all the actors purporting to be women actually are ___ ['women'].
So what went wrong with the Stiff Luv sentence? The smaller problem is that what is presumably the antecedent NP is these women, but the elliptical NP is not these women, but women. This can be fixed, in something like
In the photo, I thought I saw women, but the actors weren't ___ ['women'].
but there's still a problem, and it's a semantic problem rather than a (purely) syntactic one.
Many sorts of NPs can be understood referentially, picking out referents, in some contexts but predicatively, predicating some property, in others. In I saw women, the NP women is referential (and you can ask which women I saw), while in They are women, the NP women is predicative, predicating the properties of being adult and female to the referents of they (so that it would be puzzling to ask which women they are); similarly with Kim is a linguist (vs. I met a linguist), Kim is the president (vs. I know the president), etc.
In the Stiff Luv sentence, the antecedent and anaphor don't match semantically: the antecedent is referential, the anaphor predicative. The sentence is a play on words, giving readers a sense of having their minds twisted around.