On a car with fuel injection there can be two fuel pumps, one high-pressure and one low-pressure. The low-pressure pump moves the gas from the tank up to the high pressure pump. The high pressure pump then raises the pressure to a level that will be useful to the injectors. The two PIDs reflect those two different systems.
PID Description Min Max Units
0A Fuel pressure 0 765 kPa (gauge)
22 Fuel rail pressure 0 5,177.265 kPa
23 Fuel rail pressure 0 655,350 kPa
You can also see that the range of values returned for each is scaled to be appropriate for the range of pressures to be measured.
(I took those PID values from Wikipedia so trust, but verify.)
Edit: I added an entry for PID 23 as mentioned in comments and adjusted the layout of the max pressure column to highlight the difference in the scales of the data. In OBDII there is no way to specify the units of measure for a given PID, that is part of the reason there are two PIDs for fuel rail pressure. PID 22 is appropriate to the pressures for injection into the intake manifold, while PID 23 is better matched against the pressures in diesel or direct gas injection.
To address the other points in the question. Only PID 22 specifies that it is measured relative to manifold pressure, I would not expect that the other two would thake manifold pressure into account since they are not really affected by it.
These pressures are all controlled by regulators so that any of these PIDs should be fairly constant and not affected by load in a well running engine. Low fuel pressure would be the result of failure or clog in the fuel system.