Actually, the theory is that the different strands are by very different writers with different agendas.
https://www.google.com/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#safe=off&q=bible+with+sources+revealed
The theory is not limited to the Pentateuch -- or to the Hexateuch, for that matter.
Anonymous
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Four strands of thougtht combined to develop the Pentetuch we have today. Distinct points of view can be found. Harmonization of theme did not require harmonization of details leading to apparent contradictions.
The legal and narrative concerns of the four strands makes understanding the purpose of the Pentatuch more readily accessible
Moi
The documentary hypothesis (DH), sometimes called the Wellhausen hypothesis, proposes that the Pentateuch (the first five books of the Bible) was derived from originally independent, parallel and complete narratives, which were subsequently combined into the current form by a series of redactors (editors). The number of these narratives is usually set at four, but this is not an essential part of the hypothesis.
http://en.wikipedia.org/wiki/Documentary_hypothesis