Export Data Field Order

I've created a form with a number of text fields, pull downs, check boxes, etc. It is saved from Acrobat as a "Reader Extended PDF" so users can enter data and save.

When I use Acrobat to "Merge Data into Spreadsheet" the field order seems bizarre. It's not alphabetical and it's not based on the tab order.

Anyone know what determines the field order?

Thanks.