Is it never what you think it is?
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.