XQs....The quick answer as to why Christian books give the date of Jerusalem's destruction in 587/6 and the beginning of the 70 years in 609 or 605 BC is that they don't view the 70 years as starting in the year Jerusalem was destroyed. The Jeremiah prophecy refers to 70 years of servitude for the nations "round about" (i.e. not just Judah but also the surrounding nations) and 70 years for Babylon (i.e. as world power). It is thus thought that 70 years of Babylon as world power began in 609 BC at the final defeat of Assyria (609-539 BC), and the servitude of Judah and the surrounding nations began long before 587/6 BC -- at least by 605 BC for Judah, Moab, Egypt, Syria, etc. and 609 BC if you include Assyria.
I think tho that it is a mistake to insist too strongly on a literal interpretation of the 70 years as this was a genuine prediction, not a vaticinium ex eventu, and thus did not have the foresight of history. As such, it should not be privileged as a chronological datum superior to direct evidence of the period. As PP said, it is also a symbolic/mythological number. One should also be careful to distinguish the original prophecy from its later interpretations (or reinterpretations) by the author of Daniel, Josephus, etc.