Difference in the data between Amplitude vs Snowflake / Redshift
The Event Time in both Snowflake and Redshift is in UTC timezone. The data shown in Amplitude charts, depends on the timezone settings for the project. If the timezone settings for the Amplitude project is different then UTC then it is likely there will be differences in the data shown in Amplitude charts compared to the data in Snowflake or Redshift.
Also, make sure you are comparing the same queries, for example if you query data for Event totals in Snowflake, also make sure to check the Event Totals tab in the Event Segmentation chart. You might find the following documentation helpful, SQL in Amplitude.
Difference in the data between Amplitude vs Export API
The Event Time in the data exported using the Export API is in UTC timezone. The data shown in the Amplitude charts, depends on the timezone settings for the project. The specified date range for the export query is the time of when the event data was uploaded to Amplitude servers (see server_upload_time definition).
Thus, it is expected that there will be differences in the data shown in Amplitude charts compared with the data in from the export API.