Is it never what you think it is? In the Sappurisa Sutta, MN 113, Access to Insight it says:

'The Blessed One has spoken of non-fashioning even with regard to the
attainment of the dimension of neither perception nor non-perception,
for by whatever means they construe it, it becomes otherwise from
that.'

Or as translated in The Middle Length Discourses, page 912

'Non-identification even with the attainment of the base of
neither-perception-nor-non-perception has been declared by the Blessed
One; for in whatever way they conceive, the fact is ever other than
that.'

and by Tang Huyen, alt.zen

'Non-identification-with (a-tam-maya-taa, the state of
not-being-made-up-with-that) even the place of neither notion nor
not-notion has been spoken of by the Blessed One; for what and what
(yena yena) they think it (maññanti tato), it becomes otherwise (ta.m
hoti aññathaa ti).'

The phrase in the second translation: &quot;for in whatever way they conceive, the fact is ever other than that&quot;, seems to apply generally, as if describing Maya.
The other translations could be read as saying specifically the &quot;attainment of the dimension ...&quot; or 'Non-identification-with' should not be construed.
Should the more general sense apply?  Is that the original meaning?
Thanks to ChrisW for additional clarifications.
In my understanding, this is a reference to a folk poem-turned-proverb popular at Buddha's times about impermanence and the futility of human condition:

Yena yenahi maññanti,
Tatotassa hi aññathā;


In whatever way you think of it
It invariably gets otherwise.

Here Buddha creatively uses it to emphasize the point he's making about confusing any particular meditative state with Nirvana. However refined, any meditative state is fleeting, &quot;it invariably gets otherwise&quot;. In whatever way one tries to grasp it and make it into something one can own and be proud of -- true Nirvana remains unreachable to the grasping mind. It cannot be held onto, cannot be conceptualized, it can only be realized through actually letting go, or, in Mahayana terms, through embracing Shunyata as one's actual modus operandi.
You can see on my example right here in the previous paragraph that, indeed, as soon as you use a positive statement to describe Nirvana, your very words become a conceptual box limiting the vast open space and misrepresenting the living realization.
Or to paraphrase OP's apt question title, Nirvana is never what you think it is.
To answer your question directly: the poem speaks about impermanence of everything in life in the general sense, but when the Buddha quotes it he refers specifically to the impermanence of meditative states vs the letting go of grasping at conceptual boxes (atammayata) as the only true Nirvana.