Manual probably needs an update then, I suggest putting that in as a bug report.
The reason it doesn't matter if there is only one flight is because of the way the pax are assigned. If I'm flying an a380 on a route, supplying 800 seats, and you're flying a 50 seat prop, and the daily demand is 40 pax, we should get about 20 pax each. A bigger plane won't make a difference, a smaller plane won't make a difference (as long as it still has enough seats). As long as we're supplying 1 flight each, we should each get about half the total pax.