The answer is that they simply do not have the same muzzle velocities. The 6.5-284 has considerably greater case capacity, holds more powder, and at equal pressures has a higher velocity. This higher velocity keeps the bullet velocity above the speed of sound at 1000 yards, a critical factor in long range accuracy.
Another factor is twist rate. The very long .264" bullets need a fast twist, and some .260 rifles may not have a fast enough rate to fully stabilize the very long .264" bullets.
That all said, the .260 can be loaded to do the deed at 1000 yards, and is among the common cartridges used on the 1000 yard line at Camp Perry and other ranges - it's just "reportedly" easier to do it with the larger 6.5-284.
If both are used in a short action gun then you are probably closer than you know. The only big advantages with a 6.5-284 come when you use a longer action thus are able to use more of the case for powder. Without buying a fullup custom barrel all of the .260 Rems I know of come with 1-9 twist.....which...at least in my gun are suitable for most 160 gr. bullets and all 140 gr. bullets.
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