Realview supports tracking three different ways:

Google Analytics
Third party tracking such as Nielsen and Omniture
Realview database

Each tracking mechanism employs different algorithms and methods for recording activity on your digital edition.

There will be differences in reported figures for each of these methods. Generally there should not be more than a 10% variance, however different factors will determine the actual differences.

Why are they different?
There is no one reason as to why the figures are reported differently, but here are some examples of potential reasons:

Session times vary
The definition of a session varies across the tracking systems. A session is the amount of time that could determine how many visitors have been to look at a digital edition. For example, a session on Google may be 20 minutes where as in Realview it is 25 minutes. If a visitor clicks on one page of your digital publication, then has no activity for 22 minutes then clicks on another page, Google considers this as a new visitor where as Realview would consider it the same visit.

Some tracking requires a page refresh
The Realview viewer employs AJAX which means that the web page gets updated with being refreshed. Sometimes tracking systems rely on pages getting refreshed to track activity. The Realview database tracks all activity.