Different lots of powder can have a slight difference in performance. And, let’s not forget that different groups/brands of brass and primers can generate different pressures. Also, atmospheric conditions can affect tests on different days. Not to mention, it’s not unusual to see different load suggestions from different sources. I’ve noticed that Speer loading data is usually mild compared to other sources. To be safe, start out light, and carefully work your way up. If you start to notice hard case extraction or primers starting to flatten or push out, then you are too hot.
Indeed, but how do you get one source that has a starting load considerably above another sources +P load? I could buy a small difference, but that is an Atlantic ocean of difference.