Here's a textual puzzle by way of illustration: David and Goliath. Look at the following passages. (Both quotations are from the ESV.)
|Saul sent messengers to Jesse and said, “Send |
me David your son, who is with the sheep.” … And David came to Saul and entered his service. And Saul loved him greatly, and he became his armour-bearer. (1 Sam. 16:18–21)
|Saul said to him, “Whose son are you, young man?” And David answered, “I am the son of your servant Jesse the Bethlehemite.” (1 Sam. 17:58)|
The text critics have an easy way to solve this, of course. Take Tov's Textual criticism of the Hebrew Bible (p. 336). Having proposed a (fairly typical) interweaving of two different traditions as the explanation for various features of the text including the points I have just raised, he writes,
The editor of M T S V — who joined version II to version I — apparently with the intention of preserving a parallel ancient story, failed to take into consideration the contradictions that were caused by the combination of the two stories. (src)However, I fear that Tov is expressing rather too much confidence in his own abilities, and rather to little in those of his subject. For the sake of argument, let's agree to Tov's idea of interwoven traditions  and say that the redactor of Samuel (let me make this person male, because it's almost certainly accurate) was taking two previous traditions and using both for the basis of his text. Then clearly he thought that they worked well together and made a coherent story which was worth telling. Remember that on this assumption, he didn't just sit them next to each other; he actively moved bits of the narrative around because he thought it a worthwhile exercise from the point of view of his readers.
Then along comes one such reader, telling him he got it all wrong and missed a glaring 'contradiction' in the text. That sounds fairly unlikely to me: if it is as bad as all that, wouldn't he also have spotted and corrected it, or if not him then some scribe later on?
(I suppose at this point, I ought to suggest that this section of Textual criticism was in fact written by at least two different authors, let's call them T and ur-Tov. The person who put it together [let's call them Tov] clearly didn't think through the contradictions produced by combining T and ur-Tov in this way.)
It seems to me that Tov is presuming to tell the redactor how he should have written his text. I'd prefer it if textual criticism contained enough humility to recognise that generally, redactors get it right, and to proceed on the basis that even when we can't see it, they knew what they were doing. It'd only be charitable, especially given that the tools of text criticism are dangerous and liable to rebound at any time.
None of that explains what that first scribe who wrote Samuel was thinking, of course. And I'd like to know, because it's puzzling me greatly! But it's got to be a first, tiny step towards explaining it if we recognise that he was, in fact, thinking.
 Interwoven textual traditions is the standard text-critical way to answer these questions. It forms the basis of an entire theory of how we got the Pentateuch. I'm not saying it's always wrong, but it does seem to be somewhat over-used.