I think Skips OP was aimed at the fact that just because data might be for a certain bullet weight, not all bullets of the same weight are of the same length. And yes that can have an effect on pressure.

As others have stated there other factors that will also effect pressures, such as bearing surface, bullet composition and hardness, temperature, chamber dimensions, barrel fouling and etc.

But let's look at the data given both in print and on the 'net. Most of them don't give a clue as to what any of these variables are. They list a lead bullet of a certain weight with no information of seating depth, composition, bearing surface or etc. And I know they are well aware that there are people out there that don't care to find out either, they will use it regardless starting at the max load. And it usually doesn't cause a problem. Usually, but occaisionally, in a worst case scenario, some poor fool will blow his gun up due to a lack of caution or attention. Always start at the starting loads if you are using any component other than the data was derived with. Inculding bullet, primer, firearm etc. It you read their disclaimers this is what they use to save their butts.

And a word of caution too. Just because you aren't getting the velocities a data source lists for a certain bullet, doesn't mean you aren't getting the pressures. This isn't a green light to add a little more powder until you get the same reading on your Chrony at the local range as they did in their lab.