A systems architecture is the core of a system. It is the basis on which the entire system stands. Hence, it is under development through the entire process. You see as the developers incorporate more items into the main core, they have to fine tune and eliminate the problems in the design. They do so by introducing changes in the main core.
This is because as the project proceeds, the developers understand more about the different aspects of the project and the development team initiates the necessary changes needed to support the purpose. This ensures that the final product does not contain bugs.