This is a good question. It comes down to the underlying structure of those chapters as they fit together. Thoreau condenses his two-year experience into just one here, and he uses a standard seasonal approach, beginning in summer and ending the following spring. The progression of the seasons lends coherence to the text as a whole. He also introduces his most basic information in a sequence throughout the book – in spite of how long and cumbersome the first and introductory chapter, “Economy,” is. In “Economy” he tells us:
I lived alone, in the woods, a mile from any neighbor, in a house which I had built myself, on the shore of Walden Pond, in Concord, Massachusetts, and earned my living by the labor of my hands only. I lived there two years and two months. At present I am a sojourner in civilized life again.
In the second chapter, “Where I Lived, and What I Lived For,” he shares his purpose:
I went to the woods because I wished to live deliberately, to front only the essential facts of life, and see if I could not learn what it had to teach, and not, when I came to die, discover that I had not lived.
The intermediary chapters are filled with daily life adventures and occupations, as well as an inventory of the site, the ponds, the animals, the village, and in “Sounds,” the nearby railroad line. Then he wraps up everything nicely in the “Conclusion:”
I left the woods for as good a reason as I went there. Perhaps it seemed to me that I had several more lives to live, and could not spare any more time for that one. … I learned this, at least, by my experiment: that if one advances confidently in the direction of his dreams, and endeavors to live the life which he has imagined, he will meet with a success unexpected in common hours.
So, Thoreau DOES craft a beginning, a middle, and an end to this book. And his deliberate choice to whittle two years into one makes the presentation not only more manageable to the writer, but more accessible to the reader as well.