Garmin Race Predictions

I am training for a marathon this Fall. To Trai. All I did was enter the marathon into the Garmin Callander and I am now just doing the watches recommended daily workouts. So far it is going well. One question I have is the Race Prediction chart is shows my 5k/10k/hm/m times... The 5k/10k are pretty close to my PRs. However the race time prediction for the marathon it gives me under race predictions is a different time than the race time prediction it gives me under the event option for the actual half marathon. The race predictor is 4:38 and the predicted time under the scheduled event page is 4:22. Anyone know what the difference is in these predictions?