Bullet seating depth and temperature will effect velocity but in both cases the differences are too small to make that much difference.Were the powder ,primers and bullets from the same lot #s on both days?How were the light conditions on both days?Some chronographs are effected by lighting conditions.It may seem silly,but is your chronograph a chrony?I have witnessed someone using one that wasn't totally opened up to the stops.The result is noticeably higher readings.