When people say Jyotish is not a science, This is true to an extent, as while learning the subject, there are multiple facets of the subject. Everyone has a different understanding of science and Jyotish, but it takes years to practice, master and reach a point where you start understanding the cosmos in the birth chart.
One of the secrets lies in Vimshottari mahadasa on how the years have been assigned to dasha, such as Shukra dasa is twenty years long and the Sun is only six years of Mahadasha; the timing is the time taken by the human body to adjust to the planetary atmosphere and complete cycle of a planet If you have not read these blogs- I would suggest going for them.