Ref: Fuel freezing point, Actual vs Spec
A specification is set out by the purchaser or authority over an industry for any given chemical substance (including aviation fuel). The specification sets out for some parameters the lowest acceptable quality of the substance, and for other parameters the widest acceptable range. Having worked in the industrial chemical business for the past 14 years, I know it is not unusual for a chemical substance to be of a far higher standard than that set out on the Specification Sheet.
The reason the spec is put together this way is to allow for variations of product characteristics due to slight differences in manufacturing processes, slight differences in constituent products, slight differences in ambient conditions during the manufacturing processes and various other deviations during manufacture. Someone more familiar with the cracking of hydrocarbon fuels will no doubt provide the specifics of these variances and their corresponding tolerances within the spec.
In this instance, the lowest acceptable quality of the fuel is a freezing point of -47C, but the fuel itself has an actual freezing point of -57C (therefore it's quality is above spec). While this does not rule out the presence of a contaminant, it is certainly not a positive indicator of one.