Not every representative metaphor is a well-known proverb. A writer may use a complete image for a new situation for the first time, creating representative metaphor without the expression becoming customary among speakers.
Criterion of representative metaphor: a whole pictured situation is applied to another on the basis of resemblance.
Additional property of a proverb: the utterance becomes conventional and readily recognizable in subsequent contexts.
Thus the stepping-forward example is primarily explained through composite likeness, while the “milk in summer” example also illustrates established proverbial form and its preservation.