So then one band just provides less force. If it is that simple why not adjust the bandstretch instead?
I am thinking, when you lower the band stretch, you also shorten the power stroke. For the same load on trigger mech, you get less velocity, and carry more ballast. So, maybe this is the reason for thinner band. Or perhaps, not just extra weight, but handling, tracking fish etc. Perhaps some empirical "feel" for it? Kind of a sweet spot for the particular barrel/shaft/bands combo?