In the case of a poetic passage (Deuteronomy 32:43), the medium length Qumran text constitutes a halfway house between the long Septuagint and the short traditional Masoretic version.

Septuagint Greek

Rejoice, O heavens with him and let all the angels of God worship him.

Rejoice, O nations with his people 

and let all the sons of God declare him mighty.

For he shall avenge the blood of his sons

and shall take revenge on, and pay justice to, his enemies

and shall reward them that hate him. 

Qumran Hebrew

Rejoice, O heavens, with him and all you "gods", worship him.

For he shall avenge the blood of his sons

and shall take revenge on his enemies

and shall reward them that hate him. 

Masoretic Hebrew

Rejoice, O nations, with his people;

for he avenges the blood of his servants

and takes vengeance on his adversaries. 

In general, biblical poetry allows for more textual freedom than prose, whether it is in the Hebrew itself or in Greek or Aramaic translation. The hand of the ancient Jewish scribe of the Bible was not tied. These scribes were not servile copyists but felt entitled to exercise creative liberty. Flavius Josephus also declared that in retelling the scriptural story he did not add to or omit anything from the Bible. In fact he was regularly doing the opposite to convey what he thought to be the correct meaning of the text. 

A biblical text differing from what has become traditional may have two causes. Either the scribe (or the Greek or Aramaic translator) employed a model, which was not the same as the "official" version. Or the scribe/writer deliberately re-edited his prototype and created a new genre, the "Rewritten" Bible.