For Paul in Ephesians 4:8, clearly there are some changes from the text that Paul is supposedly citing which is Psalm 68:18. Comparing Psalm 68:18 and Ephesians 4:8, it is obvious there are differences. I believe Paul was actually using the LXX but has changed its words for the occasion in Ephesians 4. The LXX version of Psalm 68:18 and Ephesians 4:8 differs by a few words: Psalm 68:18 (it is actually Psalm 67:19 in the LXX): ἀναβὰς εἰς ὕψος ᾐχμαλώτευσας αἰχμαλωσίαν, ἔλαβες δόματα ἐν ἀνθρώπῳ , Ephesians 4:8 ἀναβὰς εἰς ὕψος ᾐχμαλώτευσεν αἰχμαλωσίαν,* ⸆ ἔδωκεν δόματα ⸀ τοῖς ἀνθρώποις. Structurally you can see the LXX and Eph 4 is similar but for a few words changes. The first change is from 2nd person (LXX) to 3rd person (Eph 4) with regards to who is doing the ascending in the first line. So the English Bible rightly translates the phrase to "When he ascended on high" versus Psalm 68, " you ascended" ...