Rookie Reloading Mistake
Not to hijack a new reloader's thread or to bring up an old one of mine, some may recall the wild swings I was getting when I started reloading 12 gauge with an MEC Sizemaster (pretty much a beefed up MEC 600 Jr.) Powder drops often varied by a half a grain and sometimes a whole grain. I think I found the problem when the snake bit me.
Some shot had found its way into my powder. How I don't exactly know, but it was there. I wonder if the insert on the top of the charge bar needs replacing? Anyway, I am proof of Murphy's Law.
So if you are getting wide variations in powder drops by weight, you might want to look for shot in your powder.