The point is that the first author with zero out of six LGBT characters will just point to the statistics and say, "Well, the chances are slim—16.7% to 39%. The dice just didn't fall that way. Can't blame me." And so will the second author. And so will the third. And the fourth. And you end up, in aggregate, with much lower than 3-8% representation, because every individual author can be sure that the low odds won't make them look bad.