You raise an interesting point. There is the difference between two gears where you can shift between them. Then there is the difference between two gears where you have to stop and change your equipment and then get on the bike again. I have no proof, but I'd say the former can be smaller than the latter for it to be perceptible. If you're riding a SS, you're going to be riding in conditions where, if you could shift, you would shift but can't. So a percent here or there can't make a practical difference in my mind.