It is related to the ballistic arc of the bullet; with the gun zeroed at 50 yards, the first time the bullet crosses the line of sight is at that point. With the line of the bore 2 inches below the line of sight, it is lower and still on its way up to the line of sight at shorter distance. Beyond 50 yards, the bullet is actually above the line of sight, and re-crosses the line of sight near 200 yards.


Your deer rifle has a similar arc, but with a 200 yard zero, the bullet has risen above the line of sight already, and is recrossing at 200 yards, hence the need to hold higher for 300 yards, and lower at 100 yards.