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I assume it is a 60 Hz system and you are simulating with absolute rotor angles.
A system phasor (clock) will rotate 36060 = 21600 degrees/s. For a 50 s simulation it will be 2160050 = 1080000 degrees.
The default in PSSE is absolute rotor angles. The change in absolute rotor angles is the integral of the frequence deviation from nominal frequency. In your simulation the change is about 0.25 degrees. The deviation from the angle of the syncronouos clock is 0.25/1080000 = 0.00000023148148 pu.
This corresponds to an average frequency deviation of 0.00001388888 Hz. The average rotor frequency during the simulation is thereby 59.9999811 Hz. Pretty close to 60 Hz in the flat run simulation!
Your result clearly demonstrates that dynamic simulations are not initiated with zero mismatch and for example the rotor speed deviation is not exactly zero during a flat run simulation.
It is better to select Dynamics - Simulations options... - Simulation - Set relative machine angles. This "problem" with a small change in rotor angles will then not be seen.
If you zoom into the other parameters you will see that they are not exactly constant. There are some very small changes in a flat run simulation, but they might not be seen in a normal plot.
