Why is 30 the default length for VARCHAR when using CAST?

Question

In SQL server 2005 this query

select len(cast(\'the quick brown fox jumped over the lazy dog\' as varchar))

returns 30 as length while the

Answer 1

My theory is that default 30 character length originated from the U.S. Postal Service specs for name and address lines:

http://pe.usps.gov/cpim/ftp/pubs/pub28/pub28.pdf