A centralized system can always be considered a degenerate case of a decentralized system in which one node has acquired more power than other nodes, or even in which no other nodes exist.
However, a decentralized system can never be considered a special case of a centralized system.
Ergo, a decentralized systems architecture can morph into more deployment structures than a centralized system can. Arguably, many scalability problems (performance, ability to innovate, ability to include third parties etc.) of today's web applications arise from this.
InfoGrid was designed from the ground up with a decentralized architecture in mind. The XPRISO protocol, which is at the heart of federating InfoGrid Nodes, is a peer-to-peer (P2P) protocol in which both sides of the communication channel have equal rights.
Further, InfoGrid's use of the Store abstraction enables even centralized InfoGrid applications to store their data in a distributed fashion.
Of course, InfoGrid applications can be, and often are, deployed in a fully centralized fashion, just like any other web application.