This puzzle has been around for many years. Some years ago, Tradition had a thorough analysis of this question but the answers were still not satisfactory. Rav Shimon Schwab z’tl once wrote an article on this (I think it it was in the sefer hajovel for Rabbi Breuer z’tl) and he maintained that the Chazal intentionally “fudged” the years of bayis sheni ,so that the “choshvei haketz’ (people who tried to guess the arrival of Moshiach ,based on various calculations-as per sefer doniel- or remozim in the torah- “bezos jovo aharon”) would not be able to accurately know the dates.
First and foremost was the reason that if you knew the exact date of Moshiach’s arrival (“Be-ito”) then you would give up hope for an early arrival and this, we are prohibited to do (ani maamin…).the other reason was-as one poster alluded to-that, in this way, we did not know exactly when the “six thousand years’ of existence for the world (see perek chelek)finished.
This was the gist of Rav Schwab’s article. For the record, it is not easy to dismiss the actual historical facts of the secular historians, who have plenty of evidence- archeological and historical- that the churban bayis horishim was on 586 BCE.