The register model maintains a mirror of what it thinks the current value of registers is inside the DUT. The mirrored value is not guaranteed to be correct because the only information the register model has is the read and write accesses to those registers.
The register model takes every opportunity to update its mirrored value. For every read operation, the mirror for the read register is updated. For every write operation, the new mirror value for the written register is predicted based on the access modes of the bits in the register (read/write, read-only, write-1-to-clear, etc.). Resetting a register model sets the mirror to the reset value specified in the model. A mirror is not a scoreboard, however; while a mirror can accurately predict the content of registers that are not updated by the design, it cannot determine if an updated value is correct or not.