From memory, the FMC averages the wind at the current level in a linear fashion from the actual wind at the present position to the forecast wind entered in the route data legs page at a position 200nm ahead of the aircraft, and uses the forecast winds entered from that point on.
It would make sense if it was assuming your step altitude wind applied for the entire sector, and was comparing that with half an hour's actual wind, averaged down to 300/90 for the remaining 3 and a half hours.
