There are lots of subitle things that change POI, such as temperature, altitude, barometric pressure, wind, wind direction, bullet weight, velocity of bullet, ballistic coefficent, line of sight down the scope, mirage....... list is a long one maybe missed a few but who cares there's enough here to worry about....
More than just point and shoot, this is what makes long range intresting, to get a hit you gotta read into all these things and the varibles involved...
heavy reading is right..........