Performance evaluation in terms of channel MSE and BER
From the simulation section of the paper:


  The SER for each Monte Carlo simulation is calculated as follows: only
  when the activity state and the transmitted symbol are both detected
  correctly for a certain user, the corresponding user’s information is
  considered to be correct, and all symbols transmitted by the user
  whose activity state is mistakenly detected are considered to be
  incorrect. To provide a benchmark for performance comparison, we
  consider the oracle LS by assuming the actual active user support set
  known at the BS.





  However, the SER performance gap of the proposed joint CE and MUD
  framework and the oracle LS always remains. This is mainly due to the
  CS-based sparse signal reconstruction algorithm with non-orthogonal
  training matrix suffers significantly, so that the data detection
  error is the main cost of user activity detection.


The non-oracle receiver has to figure out who is transmitting and hence can do additional errors. The oracle knows this information already. That's why you have the difference in the SER.