  Tunneling conductance spectroscopy in normal metal-superconductor junctions
is an important tool for probing Andreev bound states in mesoscopic
superconducting devices, such as Majorana nanowires. In an ideal
superconducting device, the subgap conductance obeys specific symmetry
relations, due to particle-hole symmetry and unitarity of the scattering
matrix. However, experimental data often exhibits deviations from these
symmetries or even their explicit breakdown. In this work, we identify a
mechanism that leads to conductance asymmetries without quasiparticle
poisoning. In particular, we investigate the effects of finite bias and include
the voltage dependence in the tunnel barrier transparency, finding significant
conductance asymmetries for realistic device parameters. It is important to
identify the physical origin of conductance asymmetries: in contrast to other
possible mechanisms such as quasiparticle poisoning, finite-bias effects are
not detrimental to the performance of a topological qubit. To that end we
identify features that can be used to experimentally determine whether
finite-bias effects are the source of conductance asymmetries.
