Assuming since this is C&V we're not talking WSD here as that's relatively recent. My tall (6'0) wife's new-ish Trek WSD says it's a 58 but there isn't a 58cm tube anywhere on that bike. TT is 56. But she tried about a bazillion bikes and that one fits her so maybe the Trek engineers know what they're doing.

So to the OP's question -- I dunno, tea leaves?