CentOS - Mar 2018 - yum list "available" versus "updates", and centosplus versioning

quite possibly more a yum question than a centos question, but i'm a
bit confused about the difference between listing "available" packages
versus "updates" packages.

  short example -- listing what the yum man page suggests should be
"available" for installation, restricting myself to centosplus:

$ yum list available --disablerepo=\* --enablerepo=centosplus
... snip ...
Available Packages
kernel-plus.x86_64                    3.10.0-693.17.1.el7.centos.plus 
centosplus
kernel-plus-abi-whitelists.noarch     3.10.0-693.17.1.el7.centos.plus 
centosplus
kernel-plus-devel.x86_64              3.10.0-693.17.1.el7.centos.plus 
centosplus
kernel-plus-doc.noarch                3.10.0-693.17.1.el7.centos.plus 
centosplus
kernel-plus-headers.x86_64            3.10.0-693.17.1.el7.centos.plus 
centosplus
kernel-plus-tools.x86_64              3.10.0-693.17.1.el7.centos.plus 
centosplus
kernel-plus-tools-libs.x86_64         3.10.0-693.17.1.el7.centos.plus 
centosplus
kernel-plus-tools-libs-devel.x86_64   3.10.0-693.17.1.el7.centos.plus 
centosplus
perf.x86_64                           3.10.0-693.17.1.el7.centos.plus 
centosplus
postfix.x86_64                        2:2.10.1-6.0.1.el7.centos       
centosplus
postfix-perl-scripts.x86_64           2:2.10.1-6.0.1.el7.centos       
centosplus
postfix-sysvinit.noarch               2:2.10.1-6.0.1.el7.centos       
centosplus
python-perf.x86_64                    3.10.0-693.17.1.el7.centos.plus 
centosplus
$

  first, i always thought "yum list available" would list only
packages that i did not *already* have installed, and if i wanted to
list installed packages eligible for update, i would run:

$ yum list updates --disablerepo=\* --enablerepo=centosplus
Updated Packages
postfix.x86_64                        2:2.10.1-6.0.1.el7.centos       
centosplus
python-perf.x86_64                    3.10.0-693.17.1.el7.centos.plus 
centosplus
$

  as you can see, the alleged updates are also listed as "available"
from the first command -- has it always been this way? if so, i guess
i just never noticed. (is there a way to list *only* available,
uninstalled packages with some sort of filter?)

  more curiously, some of the available packages from the centosplus
repo have a package version that says merely "centos", not
"centos.plus":

postfix.x86_64                        2:2.10.1-6.0.1.el7.centos       
centosplus
postfix-perl-scripts.x86_64           2:2.10.1-6.0.1.el7.centos       
centosplus
postfix-sysvinit.noarch               2:2.10.1-6.0.1.el7.centos       
centosplus
                                                         ^^^^^^

  is that deliberate? it would seem to have the potential for
confusion if one searches later for packages based on that string.

  finally, the alleged update for python-perf:

python-perf.x86_64                    3.10.0-693.17.1.el7.centos.plus 
centosplus

would seem to match *exactly* the currently installed numeric version
on my system:

  $ rpm -q python-perf
  python-perf-3.10.0-693.17.1.el7.x86_64
  $

that is, both seem to be "3.10.0-693.17.1.el7", so how would this
constitute an available "update"? perhaps there's something
fundamental about version numbering and the centosplus repo i'm
missing here.

rday