I suspect that, in the UK at any rate, this wheeze may have been originated in the mid-1960s by the navigation (performance) department of my own airline, BUA (the one that was later taken over by Caledonian to become British Caledonian).
When you are planing a scheduled operation to Entebbe or Nairobi non-stop from the then 8000-foot runway at Gatwick, using shiny new VC10s, you don't want nearly half the flights to have to tech-stop because the runway is not dry. In the 1970s, our performance instructor (George Seaton) indicated that the airline had come to an arrangement with the ARB (or whomever) that, provided the braking action was good, the screen height could be reduced from 35 ft to 15 ft in the wet-runway case, to accommodate a 10-knot reduction of the V1.
As an aside, George and his colleagues were ahead of the game on take-off performance on contaminated runways. This was arguably a two-edged sword as far as its effect on our jet operations were concerned. I well remember sitting for several hours one wintry day at Manchester on a BAC 1-11 around 1980 trying to explain to my passengers why other airlines' BAC 1-11s were departing to various destinations, whereas we couldn't depart for Gatwick.