According to logic/reason/evidence, it was not expected to be so familiar. Spelling: shouldn't
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contrapositionIs it conditional?Not really. If something "shouldn't be" a certain way or doing a certain thing, then it is not correct/logical/expected for it to be that way or doing that thing.
contrapositionDoes it mean his voice was unusually very familiar just for once?There is no sense of "just for once".
contrapositionThanks to your efforts...But I still can't fully understand.In this situation given, is his voice familiar to the speaker or not?Yes, unexpectedly so.