In activity monitor what time frame is the data sent / received data from? In the network tab in activity monitor there is a read out of data usage showing sent and received in GB. What this dosnt tell you is over what time period this is, is it from a day, a week and month ? 

This is from OSX Yosemite.
For the top two figures, since last boot; for the bottom two, current.  

After comments: It may be since launch of Activity Monitor - I have mine launch at boot &amp; it stays open all the time, so I probably didn't notice the potential discrepancy.
The little graph is 'since switching to that tab', as it doesn't log more than about 5 minutes of data.

This machine's been up 9 days, this looks about right for my standard usage - 



If I initiate a large download, the bottom figures become this [on a 150Mb/s line] - 



The graph scale auto-zooms, so the earlier few KB/s vanishes into a flat line.  

Once the file has finished downloading, the top figures show the increase of about 2GB &amp; the bottom pair goes back to being relatively idle