networking - Why do I have only incoming traffic when copying from network source to network destination?

I have a VirtualBox VM that is located on a network share of my NAS.
When I copy something from the VM's disk onto another of the NAS' network shares (so the data is visible outside the VM), the Windows 7 Task Manager shows only incoming network traffic (yellow graph). It's as if the data was only received but never sent over the network. I verified that the data arrives on the other network share.

As I understand it, the data flow looks like this:

+-NAS--------+      +-Win7 PC (VM Host)-+
| Share1(VM)-|>---->|-+                 |
|            |      | |                 |
| Share2 <---|<----<|-+                 |
+------------+      +-------------------+

If it was like this, I would see incoming and outgoing traffic, right?

What am I not getting here?