External offset voltage adjustment on NE5534?
Can you please explain what "mismatches in the differential input
  stage" is?


All op-amps have an input voltage error term. It's just like a small millivolt battery being placed in series with one of the inputs. If you build an amplifier with a gain of ten, the output voltage from the op-amp becomes offset from where you would expect it to be by 10 x the mV offset I just mentioned.

It is specifically this static error that the offset-voltage nulling circuit intends to eradicate. But, also remember that the offset voltage that is trying to be nulled, will drift with temperature (and time) and these drifting effects cannot be "cancelled" without re-positioning the potentiometer in your circuit.