Answer: The bytes per row amount is based on the disk space used for table data, extension and index pages. It doesn't include free pages in the database, but it does count free space in pages allocated to this table. As such, this number is usually larger, sometimes quite a bit larger, than the average number of bytes a query might return. To avoid showing inflated numbers for very small tables, the bytes per row is only shown for tables with at least 2 data or 2 extension pages.

See also...
The Display Schema Page - The Tables List