Word varied the number of lines in order to prevent widows and orphans (when the first line or last line of a paragraph is at the bottom or top of the page by itself).
I remember being on a panel for new writers with the great Poul Anderson. After he listened to the rest of us talk about manuscript formatting for awhile, he pointed out that while having a well-formatted manuscript was important, the story was more important, and editors would not reject a story because of things like the number of lines on each page. (For one thing, they don't have time to count the lines.)
Patrick Nielsen Hayden and other editors have said similar things. In fact, some editors have warned that sometimes writers worry more about how the manuscript looks than about how the story reads, and they are focussing their energy in the wrong place.
K.D. Wentworth has taught elementary school for many years, and she has been a writer for most of those years. She knows how it is with manuscripts and with submitting and with being human.
I've had at least one email discussion with David Wolverton about the very subject of lines and margins. He said MSWord could sometimes ba a little off like that and suggested this setting (Which has my current WIP at exactly 25 lines per page):
Left Margin: 1 inch Right Margin: 1.5 inch Top Margin: 1 inch Bottom Margin: .5 inch
His instructions were with the "line after" at 0. It's worked like a dream ever since.
Chris does have good manuscript formatting information.
[This message has been edited by InarticulateBabbler (edited September 23, 2008).]