This option (default) and its useful non-default opposite (no-convert-sk-to-pk)
are not documented in the Gnupg User's Manual.

no-convert-sk-to-pk is essential when copying both a secret key and the
corresponding public key from one keyring to another, since in its absence
self-signatures from the original private keyring are copied to the target
public keyring, in addition to the self-signatures from the original public
keyring, which causes extra signatures to appear when listing the target public
keyring.