In our experience the operational data provided through the SP-API will match what is being reported through the consoles. To caveat, Amazon will report similar data through several different reports, and it is important that you are comparing your API data to the proper corresponding report in Amazon.
For example, the Seller Central API Orders data will match the “All Orders” report in Seller Central. The “All Orders” report in Seller Central is not meant to match the console’s Business Reports “Detail Page Sales and Traffic by Date” report. Further the Console’s “Detail Page Sales and Traffic by Date” report will not match the “Detail Page Sales and Traffic Report by Child Item”.
The reason for these discrepancies can be traced back to different treatments for how Amazon applies (or doesn’t apply) Sales tax, or how orders that have yet to be shipped are reconciled across locations. For the most accurate information, the “Orders Report” provides the most granular results, and thus our recommendation for the ‘source of truth’. If you are looking for consistency between the console and the API data, we recommend using the API data set that best corresponds with the report available in the console.