The dc values of op-amp offsets can't always be taken for granted when delivering ac outputs. No device is ever exactly symmetrical for maximum positive slew rate versus maximum negative slew rate. Consequently, there's always some range of output slew rates in which the device used limits in one direction more severely than in the other. What results is rectification of the ac signal and an apparent shift of the dc offset.
This test circuit (see the figure) can check for the shift phenomenon. The accompanying table and graph illustrate the results obtained for four devices, all of different types. As frequency and slew rate are increased, the effect can be either relatively abrupt (LF412CN and NE5532N) or relatively gradual (LF358J and TL082CP).