I believe that first of all, changes in Qi are changes in the universe, and changes in climate are gradual. It is not precise enough to express these changes with a dozen words: Heavenly Stems and Earthly Branches.
Secondly, an hour is not necessarily 2 hours. According to the ancient way of timing. In summer, a daytime hour lasts for 3 hours or more. Vice versa in winter.
Third, the first, middle, and last (dividing an hour into 3 parts) of the birth hour of the horoscope are all different. Some books have explanations on this.
(Unverified authenticity)