Why wouldn’t Joseph Smith just agree with the Bible that Abraham was 75 when he went into the land of Canaan?

Genesis 12:4 gives the specific age of Abraham when he leaves Haran:

So Abram departed, as the Lord had spoken unto him; and Lot went with him: and Abram was seventy and five years old when he departed out of Haran.

If Joseph Smith was fabricating the Book of Abraham, why would he contradict the age given in the Bible?

Jeff Lindsay explains:

Multiple documents call into question the age of 75 given in Genesis for when Abraham went to the land of Canaan. The Book of Abraham says he was 62. A variety of other ages are offered in other documents, possibly because he may have made more than one journey away from Haran, as the Book of Jasher reports. Interestingly, a Dead Sea Scroll document, 4QCommGen A, suggests that he was in his sixties when he went to the land of Canaan. It’s hard to know for sure, because the text breaks off after the “six” in what appears to be an age. The translation of Florentino Garcia Martinez gives “sixty-five years,” though the “five” is a conjecture by the translator. The Babylonian Talmud suggests that his age was fifty-two years (p. 122), while the Book of Jasher says he was fifty. (Michael the Syrian in the twelfth century wrote that Abraham went to Haran at age 60, then stayed fourteen more years before leaving.)
– Jeff Lindsay – Questions about the Book of Abraham, Part 3: Does It Agree with other Ancient Texts?

Where did Joseph Smith get the confidence to provide an age different to the Bible that is backed up by other sources?

See:

Jeff Lindsay – Questions about the Book of Abraham, Part 3: Does It Agree with other Ancient Texts?

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