kernel-devel and kernel-source both provide multiversion(kernel) so zypper
tries to install multiple versions in parallel instead of updating them.

Different kernel versions are installed in different locations, but when kernel
package is rebuilt it has the same kernel version but different RPM release. So
two packages that differ only in second digit RPM release cannot really be
installed side by side - they either conflict or they contain exactly the same
files.

Binary kernel packages include Obsoletes for package rebuilds:

bor@tw:~> rpm -q --obsoletes kernel-default-devel-5.6.12-1.3
kernel-bigsmp-devel <= 3.1
kernel-default-devel = 5.6.12-1.1
kernel-default-devel = 5.6.12-1.2

which forces zypper to remove previous package rebuilds for the same kernel
version even though it normally would try to install all of them.

These obsoletes are not included in kernel-devel or kernel-source, so zypper
installs all of them. This does not fail because they are pure source packages
and contain exactly the same files which is allowed by RPM. But this is rather
confusing as in reality they are not installed "side by side" - the last
installed version overwrites all files.

This also triggers https://bugzilla.opensuse.org/show_bug.cgi?id=1171224, where
zypper fails to removed those extra packages.

bor@tw:~> rpm -qa kernel-devel
kernel-devel-5.5.9-1.2.noarch
kernel-devel-5.5.7-1.1.noarch
kernel-devel-5.4.10-1.1.noarch
kernel-devel-5.6.2-1.2.noarch
kernel-devel-5.6.11-1.4.noarch
kernel-devel-5.6.11-1.2.noarch
kernel-devel-5.6.4-1.1.noarch
kernel-devel-5.6.12-1.3.noarch
bor@tw:~> 

I'd say for consistency all kernel packages, both source and binary, should
have the same set of Obsoletes for the package rebuilds.