Hello Harold. The ROW (Runway overrun warning) considers two cases when you are airborne. It looks at the ATIS information you put in the performance page of the FMC, and uses the runway info from the FMC data base, and the planned landing weight. From this it determines where you would stop on a dry runway (brakes Hi no reverse) and where you would stop on a wet runway (max avail braking and reverse). Is these stop distances are within the available length, all is good. On finals, when below 500 feet ( might be 300, can't remember) it uses the actual weather to recalc the distances. On shortish runways and high landing weights tail wind could trigger a " if wet runway too short" or if a greater tailwind in limiting conditions, "runway too short" warning (the dry stop line is beyond the runway).
I haven't checked, but when full, and with some fuel for weather/holding etc, landing weight will be in the 380 to 390t region.
If it's chucking down pick handles, as was suggested in another thread, calcs could have been done for a contaminated runway (3mm right?) so any tailwind would have a large effect.