It's impossible to have an equal percentage of change between sprockets, since the difference between a 12-13 and a 16-17, although both 1 tooth, are not the same percentage. At some point, like the 17-18, the percentage becomes so small that only a 17-19 makes sense. With the latest 12 speed cassettes, that's where campy moves up to a 2 tooth jump, then on to larger jumps for climbing. The new 11-34 12 speed cassette still has a 19-22 jump that's a tad too large.