Pretty sure iFly only ever just gives you the bearing between waypoints, not the wind-corrected heading to fly. When in the air, just adjust your heading so that iFly's constantly-updating course predictor arrow lines up with the magenta line, and then fly that heading. If the course predictor deviates from the magenta line, adjust your heading to compensate. Using that method, it doesn't matter if the actual winds differ from the forecast--you're flying the correct heading for the actual winds.