Sorry for the incorrect link and thanks Ole for posting the correct one. Way too long a day for me yesterday.
Note that Mr. Grant has posted a reply to explain how the test was performed. The Amidol was 100% removed in order to more accurately ascertain the impurities and the percentages refer only to elements other than Amidol.
As is pointed out, there is no way to know the ratio of Amidol to impurities, but I feel it is safe to say that the ratio is sufficient to cause adverse effects to be noted in use. So the amounts of impurities are obviously sufficient to affect the usability of the product.
An interesting study for sure!