I know this is old, but since I just ran into this, and in case anyone else runs across this, the answer is because the storageType.DeviceID is set to VIRTUAL_STORAGE_TYPE_DEVICE_VHD.

Per https://learn.microsoft.com/en-us/windows/win32/api/virtdisk/ns-virtdisk-virtual_storage_type:

add: public const int VIRTUAL_STORAGE_TYPE_DEVICE_ISO = 1;

change to: storageType.DeviceID = VIRTUAL_STORAGE_TYPE_DEVICE_ISO;