Here is my 2 cents worth. Tell me if I am wrong.
First, the default coaching gameplans are pretty horrible. This is what keeps scores down and drives veteran coaches crazy. In a regular league, if you see something amiss, you just write a rule to correct it. In a Roster Only league, all you can do is roll your eyes.
Second, many of the Roster Only leagues are in their infancy. So, many key offensive players are not yet fully developed and their play knowledge is pretty low.
Third, the talent (should be) fairly even across the league until the third season. During the third season, you begin to see more separation between the good and bad teams. As the good teams accumulate better players and the bad teams begin losing them (because the coaches aren't paying attention), the offensive skill level of the good teams will increase.