Let’s assume, for the sake of argument, that every perceived inconsistency claimed by modern biblical scholars in the Pentateuch is an authentic inconsistency rather than a misperceived inconsistency. The question then arises, why do such inconsistencies exist? The first thing we should note is that inconsistencies among sources is much more common in premodern texts than in modern texts. This is in part due to scribal errors in transmission. Printed books from a single edition are always exactly the same. Manuscripts are never exactly the same. Second, ancient authors lacked any type of reference materials they could consult for details of people, facts, dates, events, etc. Jeremiah could not Google Nebuchadnezzar to find the historical background on the Babylonian king. This obviously creates consistency problems between authors, but it also contributes to consistency issues within texts by a single author.
A fundamental assumption of Documentarians is that the separate manuscripts of JEDP are essentially internally consistent in most matters. When the Redactor/s compiled and conflated these texts into the Pentateuch, however, he became completely indifferent to inconsistencies. If the manuscripts the Redactor possessed made inconsistent statements (e.g. Sinai vs. Horeb as the mountain of the revelation of the Law), the Redactor inexplicably refused to lift his reed to harmonize his final product. Thus, for the theory to function properly, one must assume fundamental internal consistency within each of the four sources of JEDP separately, while positing an utter indifference to consistency on the part of the Redactor/s who conflated these four sources.
One the other hand, if we assume—as must have been the case—that the original four JEDP sources had internal inconsistencies, and that the Redactors sometimes harmonized perceived inconsistencies in his JEDP sources, then the capacity of modern scholars to separate those sources into four originals based on perceived patterns of inconsistency becomes seriously undermined. If we assume that the JEDP sources were sometimes internally inconsistent, then some of the perceived doublets and inconsistencies scholars find in the Pentateuch must reflect internal inconsistencies in the original four sources, rather than merely assuming that the Redactor ignored and accurately copied the inconsistencies between the four sources. Such inconsistencies thus might not confidently be used as grounds for separating a text into different sources as proposed by Documentarian. Likewise, if we assume that the Redactor sometimes harmonized inconsistencies he perceived in his four sources, then some of the patterns of consistency that modern scholars perceive and try use to assign passages to the four original sources are in reality artifacts of the Redactor’s editing rather than the original sources, and are thus irrelevant to attempts to differentiate the original four source.
As Whybray succinctly put it: “the [Documentary] hypothesis can only be maintained on the assumption that, while consistency was the hallmark of the various [original source] documents, inconsistency was the hallmark of the redactors!” (Whybray, The Making of the Pentateuch 1987).