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This could very well be it. You didn't say which bullets you use, but bullets with exposed lead tips can get flattened on the tips pretty easily just by the jostling they get during shipping. Take a box and carefully measure the overall length of each bullet and you'll see some variation. For what it is worth, the weights can and will vary slightly too. Anything made by mass production methods has an acceptable amount of variance built into the acceptable benchmark for that particular product.

This is why the folks that shoot for competition go to the trouble of measuring and weighing every bullet before they load them. In that world everything has to be exact. The question is, are you loading for competition or for hunting. If the answer is hunting and the reloads still group well, no problem.

If you measure all the bullets and they are all exactly alike, all the cases are exactly alike etc... and you still have this problem then something is slipping in your dies. Make sure the set ring for the die is in place and it can't move. I actually super glue the set screw in the ring after I get it how I want to help prevent this. Bad thing about that is it is very hard to make adjustments later so you need to be sure it is right. Because I shoot different bullets in my 7mm Mag I've got 3 different sets of dies, each set to a particular load. With the cost of components these days, buying a new set of dies is cheaper that coming up with a new load.