Mostly they are saying the same thing in different ways. Some predetermine a maximum velocity with a corresponding load, others have a predetermined maximum load with a corresponding velocity. Some load data is fired in pressure guns, while some is worked out mathematically. Lee's manual is perhaps the only once which provides data for powder volume as well as weight. The more manuals you have, the more information that is available to you.
I have seen a few mistakes in the loading manuals, but only a few that will get you into trouble. For example the data in the new Hornady manual suggests loads for the .375 Ultra that are far too light, and you will never get the velocity they suggest without exceeding them. By contrast, the .458 Winchester data for the new 500 gr DGX .458 bullet is dangerous, being perhaps 2 grs over maximum (based on observed velocity) due to the long shank length below the bullet's cannelure. The load they recommend works fine with the 500 gr Interlock with its shorter shank, but its far too heavy for the long shank DGX in a straight wall cartridge, when both bullets are loaded to the same velocity. By the way, Quickload is no different, and the data from it should be treated skeptically until proven. I've found; some data is pretty good, other data is out to lunch. Regardless of the source, the handloader needs to work up new loads cautiously, particularly with untried (for him) components.
Use the manuals as a guideline rather than a bible. Your rifle is individual, and your scale might not represent the true weight of your powder charge. Start low and work up with caution. If you have a chronograph, stop when you meet the recommended maximum velocity, regardless of what the powder charge is. Get a good linear micrometer so you can check for expansion of the web of the case. A load that expands the web by .001" is often, although not always, the first indication of dangerous pressure. When working up a load, reduce your powder increments to a half or even a quarter grain once you observe that your load is compressed. Compressed loads spike pressures very rapidly and without much warning, even with slow powders, regardless of the design of the cartridge case.