Different data with changed filters

We track several websites, all using AngularJS and angulartics.

For some reason we get different results if we filter for each month separately e.g. January, February, … June or select Year and the current year.

There are pages that are not shown at all with the year filter, but show in several months.
The pages have more views than other that are shown.

Where does this come from? How do we get consistent results?