That peak pressure is maintained for only a very short distance within the chamber. Obviously for a 7,200 psi & a 11,000 psi to end u giving the same velocity to the same weight charge that difference has to be made up. Generally speaking this "Makeup" space will occupy the entire rest of the barrel, which may contain on the order of 100 times the length of that occupied by the peak. Thus it doesn't take a large increase in the pressure ""Down the Barrel" to compensate for a major difference in the peak.
One of the main concerns here is to be sure the chosen powder is compatible with the reduced pressures to insure it will burn consistently & reliably in ant temperature the load is apt to be used in, otherwise squibs & failures can occur.
Unique should burn well down to that 7200 psi figure which your results would tend to bear out. but personally I would likely not trust it much lower than that. As I recall the Federal primer is a comparatively Hot primer. This is good when working to minimum pressures. Even though "Mild" primers can lower pressures even more they can also be a major factor in incomplete ignition & combustion at the lower pressures.
