Apparently, these depedencies are mutually exclusive: >=sys-libs/ldb-1.5.5[ldap(+)?,python?,${PYTHON_USEDEP},${MULTILIB_USEDEP}] <sys-libs/ldb-1.6.0[ldap(+)?,python?,${PYTHON_USEDEP},${MULTILIB_USEDEP}] and prevent update of sys-libs/ldb. But they are the exactly the same so, sys-libs/ldb[ldap(+)?,python?,${PYTHON_USEDEP},${MULTILIB_USEDEP}] should be sufficient. Or I do something wrong?
Samba releases check for a range of valid ldb versions and exit with an error if ldb version is either too old or too new. The two dep strings simply make sure to be in the valid range.