When to tweet is just as important as what you are going to tweet. To reach the largest number of people, you should strive to tweet when a big percentage of your followers is online.
The activity analytics tool provides you the ability to analyze your followers' activity, so that you can determine the best time to tweet for maximum exposure.
We realize that not all followers are equally valuable to you. Including all of your followers into the statistics would therefore cause the statistics to be less relevant.
To prevent this, the activity graphs are based on a sample of your 250 most influential followers.