What your seeing is not that uncommon when develoing loads. I have seen it go both ways above and below what book velocities are using similar printed loads.
I generally use them as a guide and look for spikes while approaching them. If I have noticed nothing out of the ordinary I work for accuracy around the velocity range I am seeking. If I hit something that groups out well even if below printed velocities I will usually note it and then work up a bit more looking for pressures. If I hit pressure I will back down and be happy or switch to a different powder.
I have a Ruger 77 which shoots every load I have including factory around 300 - 450fps below anything listed on ammo boxes or printed in manuals for a given load. I thought the barrel was shot but it has been scoped and other than a little curd being built up, it was in great shape. My only solution was to continue to work up until I hit pressure signs with it, and this was almost 3 - 4 grs above anything the manuals listed. Now my loads are shooting to listed velocities for the bullet weights and still not hitting max.
If I were you and had a good load, I would shoot it and continue to look at similar or possibly just a tad quicker burn rate to try out another load with. I know that there are better powders out there for the purpose, but I got better velocities and accuracy from RL-19 out of my 7 mag than any other powder I tried using 162gr Hornady bullets. The best part of that load was being able to drop down to the 154gr SST and not having to change anything with it.
Mike / Tx
"Heck why would I lie, most folks don't believe the truth when I tell them"