Or… it is related to the new development model in which base packages are binaries from SLE (no bp153, e.g. in kernel-default) and community packages are from the back-port project, with a few special packages unique to Leap 15.3 (e.g. openSUSE-release marked lp153)
(please see the “openSUSE and SLE in the near future” section here https://www.suse.com/c/how-suse-builds-its-enterprise-linux-distribution-part-5/ )
which also explains why there are 3 update channels now, depending on where the package comes from.