O artigo a seguir deve ajudar a explicar:
A memória mostrada no gerenciador de tarefas é a quantidade de memória física em uso pelo processo que não pode ser compartilhada com outros processos.
Os itens a seguir devem ajudar a explicar o espaço de endereço virtual:
Definições do TechNet e do MSDN:
Private Bytes (KB). The current size of memory committed to a worker process, which cannot be shared with other processes. This corresponds to Virtual Memory Size in Windows Task Manager.
Virtual Bytes (KB). The current size of the virtual address space for a worker process. This does not correspond to anything in Windows Task Manager.
Virtual Address Space: A virtual address does not represent the actual physical location of an object in memory; instead, the system maintains a page table for each process, which is an internal data structure used to translate virtual addresses into their corresponding physical addresses. Each time a thread references an address, the system translates the virtual address to a physical address.