It can seem that way, but it's really not. There are specific formulas that can be used repeatedly.
Think of it like life, very rarely is a person's career cut and dry, there are multiple facets to everyone's work (for the most part).
The methods described just help us see the different layers.
Also, with Dashas, we can see when a person is going to do certain things.
So while there can be multiple aspects to a career when the planet that indicates that specific facet is triggered by dasha, they'll lean more towards that focus.