discrepancy in stock volume amounts at Yahoo, Google, Stock Market Watch etc.

I have a question regarding differing stock volume amounts for a specific stock reported on various financial websites. For example, let's take a look at Lockheed Martin's volume amount at 12-12-2016.

Yahoo Finance: 7 080 300
http://ift.tt/2kEcr6h

Google Finance: 7 080 271
http://ift.tt/2kE603a

Stock Market Watch: 7 079 560
http://ift.tt/2kDYfdw

and finally the Lockheed Martin's official website: 7 078 760
http://ift.tt/2kE2lSO

My guess is that it perhaps could be a result of some data providers including pre-market and after-hours trading into the calculation of the volume but still, 4 different numbers?

I hope someone can explain this. Thank you in advance.

discrepancy in stock volume amounts at Yahoo, Google, Stock Market Watch etc.