  Aims. The mass discrepancy between the observed population of double neutron
star binaries by radio pulsar observations and gravitational-wave observation
requires an explanation.
  Methods. Binary population synthesis calculations are performed, and their
results are compared with the radio and the gravitational-wave observations
simultaneously.
  Results. Simulations of binary evolution are used to link different
observations of double neutron star binaries with each other. The progenitor of
GW190425 is investigated in more detail. A distribution of masses and merger
times of the possible progenitors is presented.
  Conclusions. A mass discrepancy between the radio pulsars in the Milky Way
with another neutron star companion and the inferred masses from
gravitational-wave observations of those kind of merging systems is naturally
found in binary evolution.
