more consistent. To quote the comment in etc/Makefile
that describes how it's done:
# This target builds the kernels specified by each port. A port may
# specify the following kernels:
#
# KERNEL_SETS The list of kernels that will be
# packaged into sets, named
# kern-${kernel}.tgz. These kernels
# are also placed in the binary/kernels
# area of the release package as
# netbsd-${kernel}.gz.
#
# EXTRA_KERNELS Additional kernels to place in the
# binary/kernels area of the release
# package as netbsd-${kernel}.gz, but
# which are not placed into sets. This
# allows a port to provide e.g. a netbootable
# installation kernel containing a ramdisk.
#
# BUILD_KERNELS Additional kernels to build which are
# not placed into sets nor into the
# binary/kernels area of the release
# package. These are typically kernels
# that are built for inclusion only in
# installation disk/CD-ROM/tape images.
#
bjh21