When mounting a volume, it gets an extra ReferenceCount.

IopParseDevice() calls IopCheckVpbMounted(), which checks whether the volume was mounted before. If the volume is already mounted, then simply increases Vpb->ReferenceCount, which will then be reduced in IopDeleteFile(). If the volume is not yet mounted, then the start looping until the VPB is mounted. In this loop, the function IopMountVolume() is called, which initializes the VPB - IopMountInitializeVpb(). In the IopMountInitializeVpb() function, the value of Vpb->ReferenceCount increases. Thus, when mounting a volume, Vpb->ReferenceCount increases twice and decreases only once (in IopDeleteFile()).