On Sunday my husband Tim was interviewed on a New Jersey radio show called Evidence4Faith. He talked about the argument from undesigned coincidences, which he also discusses in talks I've mentioned here and here.
Click on podcasts at this link. If I can get a permalink to the particular podcast of his interview later, I will add it. Tim's interview begins just after the seven-minute mark and goes to about the fifty-one-minute mark.
As I was listening to Tim's examples, I was struck by all the reasons there might be for a real eyewitness not to fill out the explanation for a detail. Think for example how tedious it is to listen to someone who goes back to explain every little detail he mentions in a story. When Matthew (8:16) tells of Jesus healing many people who were brought to him one evening, it would be quite natural, if he were telling something that he really remembered, for him not to pause and explain, "Oh, the reason that it was evening is that this was the Sabbath, so the people couldn't come to him earlier in the day." He mentions that it was evening in passing, casually and naturally, as an eyewitness would do. On the other hand, were someone making up the story, there would be no point in forging the detail that the people were brought to Jesus at evening while leaving that detail hanging meaninglessly, unexplained.
Similarly, as John is telling the story about the feeding of the five thousand, it would be quite natural for him to say that Jesus asked Philip where they could buy bread if he were really an eyewitness--that is, because he remembered that Jesus did ask Philip. (Tim talks about why it was Philip in the interview.) But John himself might have had to stop and think for a moment if someone had asked him, "Why did Jesus ask Philip rather than any of the other disciples?" Presumably when John told the story, he wasn't particularly thinking about some special reason for Jesus to select Philip for the question. But if someone were forging the story as fiction, he would have a reason for choosing to use a given disciple as a character at that point in his fictional narrative, and therefore he would be unlikely to choose that character without making the reason clearer to his readers.
All sorts of such things can happen when one is telling a true story, especially a story one has witnessed. One gets caught up in what one actually remembers and drops in incidental references to small facts, which facts are to some extent selected randomly by the memory as one brings the scene back to memory. This is typical of real memoirs but not of elaborate forgeries.