SAAMI pressure is 62K PSI for the 6.5 Creedmoor and 60K PSI for the 260 Rem. Differences in the SAAMI velocities of those two cartridges are small.
I doubt Hornady is deliberately reducing it's published 6.5-284 data. More likely the results are driven by their choice of rifle, barrel length, powder, and operating pressure (or fired case characteristics/measurements since their data clearly pre-dates SAAMI adoption).
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