You have to start somewhere, lot to lot variations in the same powder can be as much as 5%. What if your barrel has a twist rate of 1:8 and the test data was done in a barrel with 1:14? The test might have been done at sea level with Federal brass and you are at 1000 metres using Remington brass. There are so many variations that you need to consider, which is why you start low and work up.
You may find you can shoot five grains over the ADI published maximum, you don't grab a new lot of the same powder or a different powder with the same burn rate and go straight to you old load. The same applies to changing brass, primers or any other component.