While testing sorting of export - I discovered that I could "select" 9 records and have only 6 of them appear in CSV output file.
Assuming current rev of generated data, you can recreate on demo:
- Find Contacts, enter 'Yadav'
- Select Yadav, Raymond (cid=85)
- Export Primary Fields
You will get an empty CSV file (no header row, no data row).
Perhaps there is some "intentional / feature reason" for this - but I don't know what it might be ??