memory - Why is the 8GB RAM I purchased only showing as 6GB?

I bought 8GB RAM from ebay. It was supposed to be 4GB + 2 x 2GB sticks. However, when I put in the RAM it only shows me 6GB.

I thought it was only 4GB + 2GB modules working, however CPU-Z shows that all the modules are being used and size of every modules is 2GB.

Did I get wrong item? Is there a possibility that item is actually 4GB of RAM and it just shows wrong results? Or indeed I've been handed 3 sticks of 2GB RAM?

Edit:
Case closed! Seller has refunded me 25% of the price I paid for the missing RAM.

Answer

You have 3 2GB modules.

The part numbers can be found on Google.
CMV4GX3M2A1333C9 is a 4GB dual channel kit (two 2GB memory modules).