Based on sales, I see wayyy more demand for 10.2-10.5mm ropes than for anything sub-10mm. The price/demand relationship only works up to a point, until the supply catches up to demand. At some point if demand is high enough, production gets ramped up to a volume that starts bringing the unit cost of production down, and competitive pressures force the manufacturers and retailers to pass those lower costs on to the consumer in the form of lower retail prices. More than anything that's why 10.5s cost less than 9.4s - they're making far more of the stuff, so it's cheaper. So in this instance, the lower demand product (9.4mm rope) ends up costing more precisely because there's less demand for it.
Thanks for that explanation!