Last week I posted that when using my .45-70 sighted in at 50 yards it was shooting 12" high at 100 yards and I couldn't understand why.
I bought a mount and put a Leupold Alaskan scope on it and when sighted at 50 it is only about 1" low at 100 so the problem is solved . Well, the problem is my old eyes trying to sight with that big shiny bead .
Now I'm getting groups at 100 yards which abouts duplicates those at 50 with iron sights. -JDL

That's one reason why some shooters prefer a wide flat post for a front sight (for round paper targets). The sun can play some mean tricks as well. Some serious shooters document the conditions - sunny, cloudy, direction of wind, time of day and direction of the sun, for EACH shot launched in competition.