I don't assume someone who believes everything Paul writes or quotes from the OT is dishonest - mostly what I assume is that most bible readers trust him to be quoting it accurately and a great deal of them aren't even aware of enough of the OT to see not only when he doesn't quote them accurately, but when he gets the entire story wrong as well!
A perfect example is Paul's talking about gifts:
Eph 4: 7
But to each one of us grace has been given as Christ apportioned it. 8 This is why it[a] says:“When he ascended on high,
he took many captives
and gave gifts to his people.”
Big error on this. Paul basis the next vital verses (9-13), which deal with "gifts" - pastors, evangelists, teachers, apostles, prophets on the above verse.
Problem 1: It doesn't say that. Psalms 68:18 is the complete opposite - in it God, not man, is the receiver of the gifts.
Small
boo boo? It can't be. Because if people were familiar with what Psalms 68 :18 is referring to, namely:
Exodus 25:1-8 - God receives gifts from men to have the tabernacle built - not the other way around.
Someone who is reading Psalms 68:17-18 and knows the Exodus story knows it's referring to the Exodus (the captives)being led to Sinai (vrs 17), where among other things, such as the law being given - the people are asked to present gifts for the building of the tabernacle!
But Paul twists it all around and is counting on the Gentile reader not to look too closely. And why should he? Gentiles back then, and even largely today - weren't and aren't too concerned with the OT in detail. So, like then, they read Paul's quoting and call it Scripture, even when Paul's words completely twist it.