Everything interacts with everything else – a project is a "system." All the elements of the project – cost, schedule, scope, risk, resources, etc. – all interact with each other in know and possibly unknown ways. Most of these interactions are non-linear as well.
Everything goes somewhere – projects are closed systems – all the elements of the project have to go somewhere. There are no left over parts in terms of cost, schedule, performance, risk, resources, etc.
There is no such thing as a free lunch – the net effort to deliver the project is the same no matter what the methodology used. The seemingly extra effort of higher ceremony project methods must be paid by someone when using agile methods.