They just work out of hubs, in Europe everything is shipped to Schiphol before being sent out to individual countries. When new computers are released, they are often sent directly from the source (Usually out of Shanghai.), however they will slowly stock a hub warehouse with units so they can deliver units more efficiently.
So depends on regional stock, if they had no units in America, they would come straight from China. They must have some units in Europe, hence they are shipping from there.
Assembly is done entirely within China, they don't make custom units as such, they just use an algorithm to guess how many of each configuration they would need (And adjust based on orders), then churn out however many of each one. Otherwise they'd take months to build and deliver.