Sir737,
I worked for an airline (and a bloody good one) for 35 years, including Check and Training on Boeing heavy metal.
For those of you who cannot read, I say again: The factors are there because the average line pilot, flying the average line aeroplane, has no/nil/nada/zilch chance of producing anything like the unfactored results achieved in certification test flying. With very rare exceptions, you have never been trained in or demonstrated your competence in the specialized techniques used. How many of you even know, theoretically, what comprises those techniques.
To seriously suggest you can demonstrate them, to the test standard, in an emergency diversion, is delusional.
This factoring all came about (as did most certification airfield performence requirements) from hard won experience.
Take "1.92" ---- this is the factored length under FAR 25 + 15% = 1.92 of the raw test figures, for a wet runway ---- the 15% being (last time I noticed, a recommendation) because some aircraft didn't stop in the full factored distance.
This has become such a screw-up in Australia, that we had certain CASA FOIs (with nil experience on heavy aircraft, let alone any serious knowledge of certification standards) wanting the factored dry field length multiplied by a further 1.92 if the runway was wet. This left only three runways in the whole of Australia where you could land a Citation Mustang, if the runway was wet.
Tootle pip!!