In FAA Land, Contaminated Runway information isn't certified, Assumed Temperature information IS certified, the Assumed Temperature Method was approved based on FAA AFM data, and as the AFM didn't have contaminated data there was no way to certify its use with Assumed Temperature.
As technology got better, the basis of contaminated runway data was never changed, its still based on the affects of contaminants during flight testing on a NASA Convair 880 aircraft, and is extrapolated for various aircraft. It is not flight tested for commercial aircraft.
There are limitations on using the data, such as...On an EPR engine, there is no easy way to check the corresponding Assumed Thrust N1, whereas you have charted values for EPR/N1 at actual temperature. On a contaminated runway you can therefore see the Actual EPR vs N1 and guard against wrong readings due to contamination/icing.
And as FLEXPWR has stated, contamination on a runway isnt uniform, so allowing fudge factors makes sense.
Mutt