OK, been a while since I've been in one, or examined how the power is transmitted through the trans, but I vaguely remember that in low gear, the first gear locks onto the counter shaft, goes to the rear down to the most rearward gear on the counter shaft and up into the main shaft. So the first gear can be the same for different low ratios because the rearmost gears on the counter shaft and main shaft are different for the different trannies, giving each trans different low gear ratios with the same number of low gear teeth.