I went to the NIH and found the name of the product and it was listed as containing 99.8% to 100% sodium carbonate.
I looked up the product's MSDS and it says the only ingredient is sodium carbonate. In fact it is listed as CAS no. 000497-19-8, which is anhydrous sodium carbonate.

NIH website page: http://householdproducts.nlm.nih.gov...nds&id=1031009
Product MSDS: http://whatsinproducts.com/msds.php?brandId=11203

Either somebody's playing fast and loose with the labeling or some website has the wrong information.