I found the following article of Aviation Week by Fred George very interesting and educative.
Approved aircraft flight manuals are chock full of precision airport performance data, numbers
obtained by some of the aviation industry’s most skilled and experienced test pilots. These data have been thoroughly verified and validated by airworthiness certification authorities to assure they accurately reflect the ideal performance capabilities of the aircraft under certain conditions specified by government authorities.
Both FAR Part 25 and EASA CS-25 state that the numbers obtained for certification purposes must not require “exceptional piloting skill or alertness.” Standard runway performance assumptions include a “smooth, dry or wet, hard-surfaced runway,” perhaps one that is grooved or porous to enhance wet surface friction; standardized relative humidity values; a regulatory 2-sec. engine failure pilot recognition delay; and flawless anti-skid braking performance, among other idealized conditions.