Why does exact balance of power …

The exact balance of power between the central and the state government varies from one federation to another due to the nature of route adopted by the country like:
(i) Coming together federation.
(ii) Holding together federation.
In the Political Sphere
(i) Coming Together Federation : In this type of federation independent units (states) come together on their own to form a union or federation. Their main aim is to form a bigger Unit, so that, by pooling sovereignty and retaining their identity they can increase their security. Federations of USA, Switzerland and Australia are the examples of coming together federation.
(ii) Holding Together Federation : Under this type of federation, the federal government decides to divide its power between the constituent states and the national government. In this, type of government the central government tends to be more powerful vis-a-vis states.
Holding together sometimes gives unequal powerto constituent states. Some states are granted special powers. Indian, Spain and Belgium federations are the examples of holding together federation