Howdy
The original question, regarding variations in data is a very valid question for the beginner. It is true, that depending on the manual, the min and max powder charges can vary a lot.
When I am starting to load a new caliber that I have never loaded before, I take a survey of all my manuals, plus whatever data I can find on the powder manufacturer's web sites.
Generally, I ignore Lee. His data is more conservative than any other manual. Some of his max loads are below the starting loads of other manuals. I take a good look at my Speer and Lyman manuals, for the specific bullets I am using, couple that with load data from the powder manufacturers, and make a good guess at a reasonable starting load. I also ALWAYS stay away from max loads, preferring to not go much above the midrange. That way, if there is an error of .2 grains in my powder measure, which is not all that uncommon, I am still down in the safe range. I shoot a lot of OLD guns, and I don't want to stress any of them, so I keep all my loads mild.
As far as cast bullets are concerned, while the seating depth of the bullet is of course very important, and the length of the 'combustion chamber' is also very important, it has been my experience that with really common bullets, for instance a .358 158 grain semi-wadcutter, or a .452 250 grain RNFP, the bullets I have bought from different manufacturers have clearly been made from the same mold. The crimp grooves are in the same place on all of them.
I do like Unique. Yes, it does tend to burn dirty, I have some 4.4 grain and 4.0 grain loads that I use with a 158 grain semi-wadcutter, and it does leave some unburnt flakes behind. But the real reason I like Unique is because it requires a larger volume than most other powders. You get less airspace with Unique than with most powders, and it also becomes very obvious if there is either 1, no powder, or 2, a double charge with Unique. Not always so easy to spot if your standard load is 3 flakes of Whiz Bang.