A trivial point, but does anyone have any idea why the line isn't always capped at exactly 80000/20000? e.g. Ian's line above is 79987/20000, and the example Weaver quoted was 79999/19999.

I've two VDSL lines, both currently at 79999/19999, but my logs show that at various points in time, one of the lines has 80000 download, and the other had 20000 upload.