In all cases, top gear is 1:1 because the drive is directly through the mainshaft to the sleeve gear, with no reduction through the gear train. The gear pair containing the sleeve gear (output to the sprocket) and its matching layshaft gear provide the drive path in all other gears, so for any given gear the total output ratio is a product of the top gear pair and the mainshaft and layshaft gear pair for that gear. For example, the stock 4-speed has a 23/18 "fourth gear" pair, and the third gear pair is 21/20, so the overall ratio in third gear is 23/18 times 20/21, or 1.22. It's a little confusing, because we call the sleeve gear and it's matching layshaft gear the 4th gear pair (or 5th gear in a 5 speed and 6th gear in a 6-speed), even though the drive doesn't go through both gears, but only the sleeve gear. Not sure where that nomenclature got started, but it's been in use for a long time. The explanation is a lot clearer if you have a picture of the power path through the gearbox, but I don't have on handy at the moment.
My format might be adding to your confusion, by showing the number of teeth in the top gear pair directly above the top gear (1:1) ratio. I guess that could be a bit non-intuitive. But it's a fairly common way of displaying the information, and it never occured to me to do it differently.
Ken