Why is there such a huge difference between volume per bar, compared to different platforms?

I don't have TC, but how does the volume diiferentiate between SF and TC?

example: 15min volume bars past couple days, SF and TOS.

Does SF include pre- and post-market volume into the first and last bars? But not only on the first and last bars do I see the difference, it's virtually every bar that's difference between platforms.

Question is, which one is correct; TOS or SF? 

I don't know why there would be a difference. We get our volume from the exchanges and the trades which have been reported.