Kudos to Patrick for pointing out what was wrong with my file--he helped me immensely, and I'm very grateful to him. The solution is: in addition to \clubpenalty and \widowpenalty, there is also \brokenpenalty, which enforces the good old typographical rule that no page should be broken after a hyphenated word. When I set it to 0, all my pages had exactly the same number of lines. Thanks to Patrick, and may others find it useful! Thomas