There are at least three possible reactions to this observation (and to similar literary devices that indicate careful construction), and each bears mention. First, a skeptic might well say that, since the details were so contrived, it follows that the story was equally contrived and hence obviously merely mythic. Second, we might infer that this merely shows divine guidance, which is obvious throughout the whole narrative. Third, we might state that while the details might not correspond to reality (because they fit a contrived narrative form), this does not mean the events did not unfold roughly as reported. The key question, of course, is whether we are to believe the Flood of the Bible actually happened, at least roughly according to how it is described in Gen 6-8. If it did, then it was caused by God, who deliberately saved mankind by saving Noah. Since we know that the God of the Bible teaches through symbols and ritual, it is not at all unusual that the destruction of the Earth and the salvation of mankind would contain much in the way of symbol and ritual. That God might direct a symmetical 7-7-40-150-150-40-7-7 structure, not just to the narrative but to the events themselves, is easy enough to concede. After all, God is said to create through a divine Word; if mere human words have a narrative structure, why would not divinely creative Words also have a similar sort of structure? Hence the second option is what believers ought to maintain. Of course, if you do not believe the Flood happened, the first option will obviously seem the right one. If you think the Flood was merely a roughly true story, embellished with myth, but with deeper, more meaningful lessons, then the third will obviously seem the right one. And then there are those who believe that God created the entire universe ex nihilo, inspired the Bible, and did not employ mythic stories in Gen 1-11; they should have little difficulty believing that narrative structure might well mirror divine intentions.