The difference is the demand level between the cities. When you launch a route with a low RI, you can only get about 25-30% of total demand no matter what you do until RI improves. With respect to ORLY (ya rly, no wai), out of your base, there's 1000+ demand per day and you are flying a diesel 9, therefore, it is possible to get near full loads on that route out of the gate. For other routes, such as to STN, where there's 200/day demand and you are flying 100 of it, in theory, your top load factor will be about 50% out of the gate in the most optimal conditions.
Competition can impact your loads, but ultimately, if you are flying a single 100 seat plane on a 1000+ demand route, you will get a fairly full plane regardless of what your competition tries to do to you. Also, the shorter a route is, the more profitable it will be, because ticket prices do not scale linearly with miles flown, even though your variable costs do.