For the majority of emails, one email will output one row in your spreadsheet, however, for emails that utilize the repeating text block or any other filters where a table is constructed, a row will be generated for each of your line items. This will mean that any single field rules (fields such as Order Number) will be copied to each row in your line items - this helps ensure data associations and gives a clear picture of where the information is coming from. An example image below demonstrates how the name and order number would be repeated for each row in the line items between two emails.
This can be switched off if needed by setting the Row Structure in your files download settings to "Append Horizontally