Since Waze uses the time you expect to be at a road segment to calculate the expected speed on that segment, it cannot use real time data when your trip time moves into a future time slot. So trips of more than 30 minutes (and on average more than 15 minutes) will include future time slots where current traffic data is not available when initially calculated. When you enter a new time slot, any real-time traffic data that is available causes your client to recalculate your ETA and reroute you at that point.