What is the difference in measurement you are experiencing..?

COAL really don't mean jack other than a reference point, bullet TIPS do vary and thus the reason to not rely on it.

The ogive measurement is what you need to know, and that's derived from a comparator that measures off the ogive.

The only concern I give to COAL is whether or not the round will function through the mag box.....

If I can allow the seating depth that my chamber likes then I don't give it another thought.