tribo, I assume that you refer to the 15% differential between the dry 1.67 factor and wet 1.92 factor.
The following notes are taken from a presentation by Transport Canada Aircraft Certification Flight Test,
Titled 'FWP 730 Operational Landing Distances on Dry and Wet Runways for Turbojet Powered Aircraft'
“There is no readily identifiable underlying technical explanation of the operational landing distance factors, which have existed for some time.
The dry factor is believed to have been derived from a statistical analysis of the variables involved as part of the ICAO Final Report of the Standing Committee on Performance. The factors have now become “accepted” as providing a satisfactory safety record”.
Comparative results; calculated vs measured/flight manual “Based on Monte Carlo Statistical Analysis”
"For dry runway the current operational factor of 1.67 for both Destination and Alternate airports appears to be reasonable
For wet runways, without reverse thrust:
1.92 for Destination airport appears to be low
1.67 for Alternate airport appears to be really low
For wet runways, with reverse thrust:
1.92 for Destination airport appears to be marginal
1.67 for Alternate airport appears to be low"
PM for more if required. alf