cn50 overnight change vs genco opening price

over the last moth, it seems that genco opens according to the cn5's change of price between 4:00 pm when market close and 9:30 am when they open. If cn50 at 9:30 am is higher than at 4:00 pm the earlier day, genco opens up. If it is lower, genco down... usually, although sometimes it is not the case