what is the difference between tamari and soy sauce?

From http://www.care2.com/channels/solutions/food/471

Shoyu is the Japanese word for soy sauce made of soybeans, roasted wheat, sea salt, and koji (Aspergillus oryzae), mold spores that when exposed to moisture begin growing giving rise to unique enzymes that create the fermentation process. This is the all-purpose cooking and condiment soy sauce made since the 1600s in Japan.

Tamari literally means liquid pressed from soybeans, originally it was the thick brown liquid that pooled in casks of fermenting soybean miso. For centuries this tamari was a rare delicacy reserved for special occasions. Eventually producers learned to brew tamari as a liquid soy sauce that had similar characteristics as the original by-product of miso. This tamari is brewed from whole soybeans, sea salt, water, and koji (Aspergillus hacho). Tamari is wheat free and popular with those who have wheat allergies.

Here's the nasty stuff. "Commercial soy sauces are made from soybeans that have been defatted with a petroleum derivative called hexane, and the fermentation is speeded up artificially. The sauces may even include genetically-engineered enzymes or hydrochloric acid-treated soy isolate, sugar and additional salt. Some light versions contain alcohol for freshness (Yamasa lite) or sugar and/or corn syrup to balance the lack of sodium (La Choy)."

Here's the site, where there is info similiar to that sent by gsiener:

http://www.msnbc.msn.com/id/15416226/

To put it simply: tamari is wheat free soy sauce.

then why does some tamari state wheat-free and others don't?