From a physical standpoint differences in viscosity can affect proportioning. Most types of proportioning equipment use some sort of orifice to control foam concentrate flow and hence proportioning rate. From basic fluid mechanics, flow of a liquid through an orifice is directly related to the viscosity of the liquid. Different viscosities will affect foam concentrate flow and may affect proportioning accuracy. It should be noted that U.L. recognizes this as a critical item, such that each manufacturer's proportioner metering orifice size is specified for the corresponding foam concentrate.
Performance is also a consideration.
This is the reason U.L. lists each foam concentrate separately with the application rate for the particular fuel group. In some cases, a foam concentrate may not be listed for use on certain fuel groups, and so the performance is an unknown factor. If the U.L Listings determine different manufacturer's foam concentrates require different application rates on the same fuel group, then surely they are not the same. This becomes a critical issue for fixed systems. Using the higher application rate of course means you need more foam concentrate which increases cost.