As you can see, the Chinese hours actually represent two-hour slots, so the 12 Chinese 2-hour periods cover a whole day.
Here is an excerpt from F. Aylward’s book “The Authentic Imperial Guide to Feng Shui & Chinese Astrology” which gives a more precise definition of the branches:
“Chinese astronomers saw the new moon as a convergence of the sun and moon. As the sun and moon seem to converge relatively at the same 12 points of the stellar dome each year, this dome can be divided into 12 equal parts which are called, among other things, the Twelve Earthly Branches. These 12 astrological signs indicate the part of the sky in which the sun and moon converge and they should not therefore be confused with the western astrological constellations.”