Perhaps it had something to do with living in a dark cupboard, but Harry had always been small and skinny for his age.

[diamondcutter]

Perhaps it had something to do with living in a dark cupboard, but Harry had always been small and skinny for his age.
Source: Harry Potter and the Philosopher's Stone, J. K. Rowling
I can't see the adversative relationship between the two clauses. Could you explain to me why the conjunction “but” is used?

 

[emsr2d2]

Perhaps it had something to do with space here living in a dark cupboard, but Harry had always been small and skinny for his age.
Source: Harry Potter and the Philosopher's Stone, J. K. Rowling

I can't see the adversative relationship between the two clauses. Could you explain to me why the conjunction “but” is used?

Rowling is saying that she (or the narrator if there is one) can't be absolutely sure that living in a cupboard is actually the reason for Harry being small and skinny for his age.

 

[Ostap]

But here is used meaning but then.