Okay, not sure I totally disagree with what you said but let's see if we can clarify some communication.
You are correct, CE and carbon content are not 'inter-changeable'. But, they are relational. Carbon is the major element determining a material's CE. No matter which carbon equivalency formula one is using. There is more than one depending upon the purpose for finding CE. Hydrogen control, hardness, re-bar weldability, etc.
Now, my reference to ambient temps is in regard to original 'pre-heating'. Many do not understand that ambient temperature IS preheat. If the ambient temps are high enough, and thus, the temp of the material is high enough, then 'further' pre-heat is not required. And, within the welding process the inter-pass temps are also regulated by not only the welding heat input but by ambient temps as well. All this was in support of my first statement that all materials require preheat at some time. I don't care what their CE, carbon content, or any other properties are, at some point the ambient temperature will require that even 1/4 X 2" A-36 flat bar be pre-heated. As is the case with ALL other materials in group I, II, or III.
Now, in Tyrone's second post he asked, "Why are these base metals combinations prequalified since one may need preheat?"
That is what I was responding to. It is not that "one" may require pre-heat. All require pre-heat. Some will require it at higher ambient temps than others. None of this is what the classification is about.
As to his original question, it is beyond my paygrade. Bottom line, they are grouped according to filler metal and base metal compatibility, usage, and many other factors that really are not dependent upon pre-heat temps being even remotely the same from one material to the next. Thus, the CE being different from one to the other is not a contributing factor when determining their placement on the table.
Oh, BTW, Scott, there are no out of date editions of D1.1. Some may still be in use from several years back if the project is old enough and specified that particular edition as being designed, fabricated, and inspected to.
Have a Great Day, Brent