Biggest discrepancy in my books is 80's vintage Win Ball Powder data. Win refused to publish above 98% case capacity, others would compress. Win would also do the muzzle up/down, +140F/-40F thing. I considered their data the gold standard of safe starting loads.
My Speer used magnum primers for their ball loads, others didn't. Hodgdon #25 list different loads for mil and win brass, others didn't allow for the different case volumes. Data from bullet companies is specific to their 150gr bullet, but most data from powder companies is generic and there are many types of 150s out there.
Add to the mix that there is a much bigger variety of bullets today. Seating length being equal, lead free bullets and flex tip FTX's take up more case volume than their lead core counterparts. Only good answer is start low, work up slow, and start low again if you change any component.
A gun is like a parachute... when you need one you usually need it pretty bad...