If you started creating reports and monitoring how much time you spent on different tasks and activities you might notice that the Reports section and the Dashboard section show different times.

Here is a small example, Dashboard shows a total time of 28h56m and Reports show a total time of 27h20m:

The reason for such discrepancy is that the Reports section shows time tracked on projects and tasks. The Dashboard section shows the computer time registered by the desktop app. 

So in Reports, you'll find your projects time and in Dashboard, you'll find your computer activities time. And as you noticed this time can be different because computer time is started to be counted immediately after you turn on your computer, and timer for tasks may be started later and sometimes not all of the tasks you were working on were added to your Timesheet.

There are a few settings that will help to adjust when computer time should be registered. Here you can learn more about computer time settings and here you can also check information about the automatic time tracking mode.