I'm not real sure where you get that velocity range. The max average pressure of 5.56 x 45mm is slightly higher than for .223 Remington. That doesn't translate into much more than a 100-150 f/s difference in most cases, assuming equal barrel lengths. Maybe 200 f/s for some strange loads.

You are correct that most ammo isn't loaded to maximum pressure. Most ammo producers load to a velocity standard, not to a maximum pressure. The lower pressures noted in more recent load data is due to the increased sensitivity of the piezo-electric pressure measurement system over the old copper crusher system. Some of the old data produced pressure spikes over the established limits that weren't seen using the old sysem.