The statistics on the dashboard and the Analytics areas work by only updating if there’s an action taking place on a campaign.

In other words, if someone opens a campaign from yesterday, the open rate will be calculated. If someone opened an email or clicked on a link from a campaign 6 months ago today, it would then calculate the open rate.

It’s not a set time period that the campaigns have been calculated for, it’s a set time period that the calculation has been available to be processed.