word choice - When something sounds too good to be true, it is or isn't?

I searcheg Google Ngrams. The vast majority of returns ended with "it is." Which is strange to me, as I used the phrase here (on EL&U) with it isn't.

In both cases, the phrase works, because the last phrase can be the correct answer to two different parts of the prior phrase in the saying.

When something sounds too good to be true, it is (too good to be true.)

When something sounds too good to be true, it isn't (true.)

When something really is too good to be true, people say "I thought it was..." or "It seemed" too good to be true, because obviously if it was true, it only appeared to be too good to be true.