Books are not released evenly across the year. Publishers concentrate their most important titles into a small number of weeks, and the bestseller lists reflect that concentration rather than any change in reading.
Retail attention is seasonal and finite
Bookshops have a limited number of prominent tables and window positions. Those positions are allocated in advance and are worth most when shop traffic is highest.
Traffic peaks before the winter holidays and again at the start of the summer reading season, so publishers push their strongest titles into those windows.
A book placed on a front table during a quiet fortnight is competing for a fraction of the customers it would have met six weeks later.
Retail buyers know this, and the placement they are willing to promise depends heavily on when a publisher proposes to release the book.
Coverage is scheduled around the same peaks
Media coverage of books also runs in seasons, with gift guides, holiday round-ups and summer recommendations planned months ahead.
An editor assembling one of those features chooses from what is being published near it, which pulls publishers towards those dates in turn.
The cycle is self-reinforcing: coverage clusters because releases cluster, and releases cluster because coverage does.
Crowding creates the quiet months
Concentration produces weeks in which several major titles compete directly, and other stretches in which comparatively little is published at all.
Those quiet periods are where smaller books can reach a list on sales that would not have registered in a crowded week.
A title's chart position therefore depends partly on what else was released alongside it, which is a matter of scheduling rather than merit.
Publishers move dates to avoid collisions
Release dates are set a year or more ahead and adjusted as competitors' schedules become known through trade announcements and retail catalogues.
A book expected to dominate a week will push others away from it, and a smaller title will often be moved rather than launched into that shadow.
Some of the year's quieter stretches exist precisely because everyone avoided them, which occasionally makes them the most favourable weeks available.
Publishers with several strong titles also space their own releases apart, since two of their books competing for the same coverage helps neither.
What this means for the lists
A bestseller list is a weekly ranking, so it measures performance against whatever else happened to be selling in the same seven days.
Reading the lists across a full year gives a truer picture of what sold than any single week does, because a single week is as much a description of the schedule as of the readers.