There are 4 curves in total: L/D solo (wingloading at 5.67 lbs/sqft) and dual (7.44 lbs/sqft). Those are read against the left Y axis. Sink rate solo and dual. Those are read against the right Y axis. As far as the bands, I suspect it's "truth in advertising" by Schweizer; i.e. "it's our calculated L/D so don't hold us to it" :-)