Here is the explanation from the ETS website (water purification section, Micropur review):
"We also inquired about the safety of using the tablet at higher concentrations, in other words, in less than a liter/quart of water, as might be the situation with an improvised water container in a survival situation. We were told that this could be acceptable for short term use. A higher concentration (less water) may result in unpleasant taste or odor. If someone has an adverse reaction to chlorine dioxide in general (which is a rare condition), this risk would increase (similar to the case with those having adverse iodine reactions). However, in an emergency situation, no negative health issues would be expected."
Water purification Section from ETS