What powder measure are you using? The reason is ask, is some types of powder measures (like Dillon) move the body of the powder measure as they cycle. In a nutshell, this means that the first several throws can be, and usually are, lighter that the final ones. This is caused, of course by the powder settling in the measure, making the powder stack on the measuring chamber denser the more you use it. Static, or non-moving measures (like Hornady) are more consistent, as long as they use a powder baffle. Still, I always "throw away" the first 10-20 throws from any powder measure if top accuracy is my goal, or if I'm using top end load data. Also, check the weights of your powder charges regularly as you load. Flake & stick powders are often the worst offenders in this regard, ball powders usually settle down pretty quickly. And, as mentioned before, powders physical caracteristics can vary from lot to lot, and every time you change powder lots, everything is subject to re-evaluation.
