How did she do this? She wrote in longhand, and cleanly, with few corrections (a page of the MS of Family and Friends (1985) is to be found online alongside the Paris Review interview) - so it was probably just a case of her allocating herself a set number of sheets of paper per chapter.
But why did she do it? She was certainly a writer, and probably a person, who lived according to her routines. Imposing such structures and patterns on the job of composition would have given momentum to a writing process that, as John Bayley says somewhere, possibly wasn't experienced at the full fever pitch of passionate engagement.
In the 1990s, in A Family Romance (1993) and A Private View (1994), Brookner experimented with chapter length. The chapters in those novels are approximately double the usual Brookner length. She returns to her old pattern in some later novels.
In the 2000s things seem to be up in the air again, matching perhaps the edgier tone of those last novels. Brookner's first and final chapters had always been subject to irregularity, but chapter 3 of The Bay of Angels (2001) is intriguing and a little disconcerting for being only four pages long.