As Ackleyman said all rifles are different, and I'd believe that's what Hodgdon's test equipment told them. A lot of things can change pressure with a rifle, and how a chamber is cut can be one of them. Hodgdon has had .280 AI data out long before Nosler standardized it. If they used a different reamer than the SAAMI reamer, they could very well have different data than other sources.

Like a said though if you aren't getting the results you want, change powders. Since mt experience with AI cartridges is limited, I feel they mask a lot of pressure signs, I'd make sure to shoot over a chronograph. I've never used the .280 AI just the regular version, but I did a lot of load development using .270 Win data and was very satisfied with the results over my chronograph.

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Use the chronograph to measure the velocity of your rounds as you work up a load. An increase in the powder charge should increase the pressure; and an increase in pressure should cause an increase in velocity. If it does not, it can indicate that some dimension inside the case or the firearm is stretching when the rounds are fired. If you increase the powder charge and do not see an increase in velocity (or worse yet, see a drop in velocity), you should stop immediately and find out what is wrong.