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The data discrepancy you’re experiencing may be due to “data sampling” in Google Analytics. When you add more dimensions to your query, the required computations become complex and Google Analytics reverts to using a sample of data rather than the full dataset to speed up operations. This results in slightly varying data when compared to the full, unsampled data seen directly on the Google Analytics interface.
Alternatively, there could be differences due to the way the API and the Google Analytics interface handle certain metrics, particularly multi-session metrics, for individuals who have had more than one session within the defined period. The discrepancy arises from the difference between a user-centric view and a session-centric view. Some sessions might not contain the data associated with the dimensions you’re pulling, which would make the session not be counted when you pull in the dimensions.
It’s helpful to note that minor discrepancies are expected when dealing with large data sets and complex queries. If precise accuracy is essential, you may need to adjust your API queries or consider a different approach. It’s always a good idea to understand and allow for potential sources of error in your data analysis.
