Why do shot groups open up over range?

Which is another reason the shots spread out over distance.

Funny thing. I stumbled on a free program called Point Blank from Hunting Nut and its results match my log to the nearest 2 inches. Perhaps I'm not having a serious velocity issue as previously convinced.

My new question is why are loading manuals so different from each other and ballistic calculators worse? One manual says I can't load hotter than 49 grains another 52. One ballistic calc says my drop is 320" the other matches my actual 420"
 
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