OP Amp as differential amplifier
There are several effects buried in that 1 Hertz output of 0.009 volts, for 2 volts input.

As was stated, the resistor tolerances matter. But this is a simulation.

The opamp has FINITE GAIN, probably only 100,000X or 100dB.

Your circuit has 40dB gain (100X).

The opamp has 100 ohms (?) Rout.

The circuit achieves (before gain) 0.009/100 = 90 microVolts Referred To Input of Common Mode Rejection at 1Hertz.