Same graph in ASME BPVC 1956 was more generous, more explicit and combined. Probably ASME changed the presentation to avoid discussion is working with plastic strain and made it more digestible...

About the software you've mentioned, I guess that their interpretation was indeed that "less than 300 F" must be a curve to the left of 300F curve, hence they thought "to the left" brings-up automatically the rule of "step 7". Understanding the reason of that rule (basically is dedicated for the linear part of curves, going to 0,0 point- not shown in graph- and "to the left" is just a visual indication) may clarify the matter. In addition "to the left" in the today graph is different from "to the left" in 1956 edition, because the older edn had more "linear" part in the graph.