It's been a minute since I've looked at that code, but I'm pretty sure the bill order is more or less the order that the game read in the recipe (the exact answer is more technical and has to do with how the game loads XML, but it's basically: mod load order, followed by file naming, then the Defs listed in the file are read top down, and finally any patches are executed. The game later builds the recipe list by grabbing all recipeDefs from the loaded xml, then the ThingDefs with a recipeMaker definition).
It's technically possible to sort the list, but I don't know of any mods offhand that do.