If any of the fields you are exporting are from a related table, and more than one related record exists for any of your records in the found set, it will export one row per related record instead of one per parent record.
You can circumvent this by creating a calculation field in your parent table for each of the related fields you want to export, which you can use to just take the value from the first related record or use the List function to get all of the related values into a single field.
Or try to ensure that only a maximum of one related record is possible from that context.
Bingo! Duh, of course! Thanks!