Разные параметры запуска JVM. Кроме того,

Server-Class Machine Detection
Starting with J2SE 5.0, when an application starts up, the launcher can attempt to detect whether the application is running on a "server-class" machine and, if so, use the Java HotSpot Server Virtual Machine (server VM) instead of the Java HotSpot Client Virtual Machine (client VM). The aim is to improve performance even if no one configures the VM to reflect the application it's running. In general, the server VM starts up more slowly than the client VM, but over time runs more quickly.