We cover this on the upcoming episode of Experimental Brewing. Here's the explanation...
The term “Broad Spectrum” when applied to a sanitizer means that it will attack a wide variety of different types of microorganisms, including gram-positive bacteria (Listeria and Staphylococcus), gram negative bacteria (E. coli and Salmonella), viruses, fungi (both yeasts and molds), as well as many parasites. Broad-spectrum germicides act on microbial membranes, cellular enzymes, DNA, and protein. Iodine-based sanitizers have been used as antimicrobial agents since the 1800s and have a broad spectrum of activity They are a powerful sanitizer in strong acidic aqueous solutions. They are generally used at 12.5 to 25 ppm available iodine, and can cause staining on some surfaces, especially plastics.
Acid-anionic sanitizers are surface-active sanitizers, but negatively charged. Formulations include inorganic and organic acids plus a surfactant. Carboxylic acids (fatty acids) are some times incorporated as well. They are unaffected by hard water or organic soils. The dual function of acid is that it can be used for rinsing and sanitizing in one step. These sanitizers must be used at low pH. Activity above pH 3.5–4.0 is minimal. Acidity, detergency, stability, and noncorrosiveness makes them highly effective. Acid-anionic sanitizers are broad spectrum against bacteria and viruses, but not very effective against yeasts and molds.
Iodophors are considered broad spectrum anti microbial vs. Star San being a being anti bacterial. The actual label for Star San lists what it is registered to kill: E Coli and Staph A – the minimum baseline for allowing a claim of being a sanitizer with the EPA. Iodophor has proven effectiveness against not only gram positive and negative bacteria, but yeast, mold, fungi and viruses and is also a sporicidal agent.