No it's NOT! It's the overall length of the bullet-case used in their testing---period! If you use a different overall length, then you're not going to duplicate their shown velocity. But more important, you're not going to duplicate their pressure either.
True, But you are not going to Duplicate their pressures by treating their data as a recipe and follow it to the letter either. Unless of course you use the same powder/primer lot number, same lot brass, same temp/altitude and of course the same exact firearm or test barrel.