Hebrews 10:5-7 contains a quotation from Psalm 40 from the Septuagint that greatly varies from Masoretic version that is present in our bibles today:
“So when he came into the world, he said, “Sacrifice and offering you did not desire, but a body you prepared for me. “Whole burnt offerings and sin-offerings you took no delight in. “Then I said, ‘Here I am: I have come – it is written of me in the scroll of the book – to do your will, O God.’”” Hebrews 10:5-7 NET
Based on the writing in Hebrews, it is commonly asserted that the author was very skilled in Greek and highly intellectual - so why the use of the Septuagint that greatly varies from the MT? Especially since the author was writing to Jewish people and knew their Tanakh well - why not utilise the MT to a Jewish audience?
I would also like to know whether there was a finalised version of the MT accessible at this time that the Hebrew writer could have used, or was the Septuagint the only accessible translation available to the writer?
And lastly, (if possible), what implications does this varied quotation have on the reliability of the NT writers and their use of Old Testament scripture?
[Not to be confused with this question about the source of the Jesus saying.]
(Also not to be confused with this question as, whilst similar, my question more focuses on the reasoning behind the writer of Hebrews choice and the theological implications behind their selection of the LXX.)