I’m thinking it’s more of a lot variation. I know it’s not a big deal to most shooting but the accuracy guys or USPSA that need to meet a power factor will pay attention when they get a new lot. It can be faster or slower.
My guess is that’s all this is. Don’t assume 155-1 is the same as 155-2.
Most manuals will tell you to go back to starting loads and work up when working with a new lot of powder. Same here.