Possibly the best thing ever said by an editor.

Indeed, Seanan's right; the periods-falling-off-the-row issue could and indeed did arise with normal sentences.

The thing, though, is that an ordinary sentence can end anywhere in a row of type, whereas a sentence that ends with quotation marks is relatively more likely, on average, to end at the physical end of a line. And the period slugs were small and skinny, whereas the quotation-mark slugs were, well, chunkier. [This is roughly the same principle that arises when you have half a shelf's worth of books on a given shelf. If the book on the "open" end is skinny, it's much more likely to fall over than a book that's wider. And for some reason, this happens more often if you have a fat book next to the skinny one....] Thus, what the typesetters were trying to minimize was the instance of periods at the outside ends of lines, and most especially periods at the outside ends of quote-marked lines.

Eventually, of course, typesetting tech evolved to a point where this wasn't an issue -- but by that time, the typesetters had worn down the grammarians to the point that the grammarians had mostly given in and decided that all right, the periods belonged on the inside of the quote marks irrespective (mostly) of other grammatical issues.