The information isn't conflicting, you simply aren't comparing apples to apples. It's obvious that either you aren't paying attention to the details or you don't believe that the differences are enough to create a change in data, either way you're not correct. All of these loads are using different barrel lengths, cartridge overall lengths, units of pressure, and most likely different powder lot numbers and all of these things will create different results. Use the manual that the bullet manufacturer sells. Hodgdon doesn't make bullets so I never really trust their data, and as I show below, they have many errors in their information (two different bullet weights fired from two different barrel lengths using the same powder charge wont produce the same velocity). Everybody complains that Hornady usually shows less powder than the other manufacturers and they assume that that means that Hornady isn't accurate but the truth is that Hornady bullets are usually shaped in such a way as to produce more pressure than a similar bullet from another manufacturer so Hornady will recommend less powder so as to not exceed SAAMI max pressure recommendations with their specific bullet.
Notice the differences between each of the loads you listed:
Hodgdon-26g max - They show the same max powder charge for a 50gr Speer SP and a 53gr Sierra HP (26gr) with the same velocity and CUP value. They list a different overall length for these two bullets, 2.210" for the Speer bullet and 2.200" for the Sierra bullet. They also list the same barrel length for both bullets, 24", with a muzzle velocity of 3200 fps for both.
Hornady-28.9 " - I'm assuming that you are referring to the 53gr HP Match bullet which Hornady says will produce 3400 fps with a cartridge length of 2.230" out of a 26" barrel.
Sierra-27.5 " for AR & 26.5 " for bolt - Sierra does a better job than Hodgdon identifying the differences between the results of these two loads. The indicate that the AR uses 53gr bullet over 27.5gr with a 2.250" cartridge length out of a 20" barrel to get 3100 fps, while the bolt rifle uses the same bullet with the same cartridge length but 26.5gr produces 3200 fps our of a 24" barrel.
Speer-28.0 " - I don't have a Speer manual so I can't really guess at which bullet you are referring to and I can't identify the specifics about the load but I'd make a guess that, just like the other loads listed above, there will be differences in barrel length, cartridge length, muzzle velocities, or other data.
The point is that the differences exist because the loads are different and whomever wrote the manual used different standards and processes for writing their manual. As I said, the safest way to choose a manual is to pick the bullet and use that manufacturer's book.
