Unusual time scheme - author's decision? editor's?

About half-way through Amy and Isabelle, and now the time frame of the novel is more clear to me. Strout clearly delineates each chapter by its season through careful description of the weather, always extreme it seems. The first chapter takes place in a sweltering summer, with Amy working the mill office with her mom, Isabelle (it's a shoe mill, we learn, the smell of glue and leather pervasive, another eerie quality in this book, as we look from our vantage upon this doomed industry that at the time, ca 1975, seems the economic engine that drives this small Maine town). At end of first chapter we learn that Amy's friend, Stacy, is pregnant. Next chapter, is it winter? late fall?, Amy and Stacy hanging out at school - I'd thought we were moving forward in time, but it gradually becomes evident that we have jumped back, that the first chapter, chronologically, should have come after chapter 7: when Amy first has intimate sexual contact with her math teacher, Mr. Robertson, and when the mill owner (Avery Chase) discovers them and, though it's not clear yet how this happens, Amy's mother, Isabelle, learns about her daughter. This explains the harshness and coldness we see, esp from Isabelle's side, in the first chapter. I really think the book should have been a straight chronology, and I wonder if an editor advised otherwise - it did no favors for a very well-written narrative. Now, halfway through, we much better understand the tensions between the two title characters - Isabelle feels entirely betrayed by her daughter, Amy, whom she had thought was staying after school for clubs etc. Isabelle feels humiliated, too - always thought she was smarter and more sophisticated than her fellow workers at the mill, and that a good mother would know if her daughter was in trouble, and now finds she's no better or different from them. We still don't know about her past and expect there's something that led her to settle in a town among strangers with her young daughter - doubt she's a widow. We also don't know what happens to Mr. Robertson, or even whether Amy knows that her mother (and others) know about their relation. The sex scene in Robertson's car is very well done and extremely creepy, nothing much harder to write than that! I do have a quibble, though, regarding plot creekiness: impossible for me to believe, in small town high school, that nobody would have noticed Robertson's inappropriate attention toward Amy. She's in his class after school every day, he starts driving her home every day, parks in her driveway where they kiss, ever day. Nobody's suspicious? Come on! Then they go for walks in the woods. And the first time they have sex in his car, something draws Avery to stop his car, get out, and see what's going on in this car parked off road mid-day? No way. But, accepting story for what it is, this is becoming quite a compelling novel.