If you read the page carefully you will see that the first 14 bytes are what's stored. The last 8 bytes are generated at SELECT time. Both of these add to 22 bytes of hexadecimal values which, when externalised as printable, occupy 44 bytes. This accounts for VARCHAR(14)
In the last paragraph at the bottom of the page is stated that 3 bytes are left for future expansion of ROWIDs - this, added to the 14 bytes previously explained, accounts for the VARCHAR(17).