I think it was just the order in which Kodak's lab techs came up with the formula. D-1 was probably a Pyro-Soda mix concocted in the 1870s. D-19, IIRC is a Hydroquinone-caustic high-contrast developer for processing lith film. Looking back, I find it amusing how much time and effort was put into seeking a holy grail of a developer. That elusive (and probably non-existent) magic soup that gave full film speed, along with fine grain, high acutance and superb tonality. Jeez guys; why not just stop when you got to number 76? It's about as good as you're going to get.