1) Make sure you have applied same parameters in Tool as well as in the GA Interface.
* While checking with Google Analytics, Move the gray circle to "Higher Precision" as shown in following screenshot.
- You can cross check the data using this simple data feed query explorer tool which works based on (Google Analytics API same like Tatvic Excel Add-in). If your add-in data mismatch with this tool, you can contact and let us know issue. If it's showing the same result as the tool then it would be sampling issue or other calculation limitation of the Google Analytics API.
When visits exceed 500,000 in any report, the data is automatically sampled. Read More