A verb the chapter does not spend freely
"God created" opens the chapter, then the chapter shifts to other verbs for light, sky, land, and vegetation — until "God created the large sea creatures and every living creature that moves." The return of that specific verb at 1:21 shows that 1:1 is not a heading over everything that follows, but the first use of a term the text is rationing.
A reader who tracks the verbs notices the gap. Between 1:1 and 1:21 the work is described differently: light is called into being, waters are gathered, the earth is asked to produce. The language of 'making' and 'letting' carries most of the middle section.
Then 1:21 brings the same verb back: "God created the large sea creatures and every living creature that moves." The habitats were shaped; their inhabitants are 'created.' The distinction is not loudly announced — it is carried quietly by word choice.
That return reframes 1:1. If "God created" were simply a summary title for the whole chapter, there would be no reason for it to reappear mid-sequence. Its reappearance suggests it marks something — that the chapter itself is distinguishing between acts of formation and something it reserves a different word for. What 1:1 opens, 1:21 resumes, and the gap between them is filled with a different register of language entirely.