OT4: Comparing analytics metrics

How Come Google Analytics Shows Different Results 

Different analytics products may use a variety of methods to collect data about your traffic, so it’s normal to see discrepancies between reports created using different products.  But the best way to think of metrics across different analytics programs is to think in terms of trends, as opposed to numbers themselves.

It's normal to have discrepancies between products, there is no need to panic.

There is no single straightforward answer to this question, and it could take several hours/ days to find an answer to this question depending on your expertise.
Depending on how you implement an analytics tool, reports may happen on the server-side or have different methods to record like metrics. Google analytics only reports on browsers with Javascript enabled and usually reports lower figures. 
Sometimes Google analytics is not on the same pages as Opentracker, which will lead to different results.
Sometime there JavaScript have different loading times, or errors on the page which can lead to different results. 
Sometimes the data is being generated for different time-zones.
It's important to note that discrepancies will always exist when counting events due to different definitions, implementations and technologies. Before you take any action, you need to first measure the level of data discrepancies. If the numbers are off by less than 5% per day, then this is to be expected.
Different analytics tools have limitations in the way they record and track data.  Compare events of one product to the events of the other product. Do not compare events to sessions, i.e. different metrics across different products. Do not compare apples to oranges.
So at times, our own CMS’ Analytics reports are more accurate than Google Analytics and vice versa.
For more accurate reporting we strongly recommend using your analytics tool and Google Analytics in parallel.  It may be difficult to compare figures between the two systems, and we suggest that you focus on trends as opposed to specific numbers.