All the reasons given above are technically correct but in a real hunting situation when the animal is in your sights and your heart is racing you do not have the time to consider all the reasons.I have memorised the word "SLOW" which stands for "shoot low" and according to my judgement of distance and angle I try to place the shot low when shooting above or below my line of sight.Any target over 45 degrees up or down I try to compensate.Minimum width of an animals chest is around 8 inches and an inch up or down will not make much difference.Thats how I go about it.