What is the difference (algorithm) between the IN/OUT Traffic counters from the <Interface View> Tab and

the "Avg" counters from the <Speed-Graph> screen (see the following pictures).

In my understanding they should be the average per selected interval, but they aren't the same (some times they are pretty close). Even for one day report there are noticeable differences. The Speed-Graph screen seems more accurate to me (the "Avg" values resulted from the Data Points seem to be correct). It's not big deal, but why the difference? 

I don't get it from the software documentation. 

Thx for your time.