Why can two nodes in a P2P network see things differently at the same moment?

Because data spreads step-by-step through a peer-to-peer network, one node can be aware of information that another hasn't received yet — giving them genuinely different views of the network at the same instant. The ripple effect that carries data across the network takes time, so the last node in the chain is temporarily out of sync with nodes closer to the source. During that window, the two nodes have different points of view: one knows about the message, the other does not. For digital currencies this matters because the network must eventually reach full agreement, even though individual nodes are briefly working from incomplete pictures.