I am very close to being convinced that there were (at least) two major editions of the Psalter. One is found in the MT and more or less reflected in the LXX, though there are numerous minor variants as well as the single major variant that the LXX includes Psalm 151, whereas the MT ends with Psalm 150. A second Psalter – a second edition of the scriptural book of Psalms – is partly preserved in 11QPs[a]; this assertion is supported by the fact that a second manuscript (11QPs[b]) and perhaps a third (4QPs[e]) also seem to exhibit this edition, whereas there is “only one scroll from Masada (MasPs[b]), and none from Qumran, whose order unambiguously supports the Received Psalter against the 11QPs[a] arrangement.” It should also be noted that 11QPs[a] ends with Psalm 151, as does the LXX.

Ulrich, E. (1999)The Dead Sea Scrolls and the Origins of the Bible. Eerdmans. pp. 30