https://github.com/ceph/ceph/pull/38654
changes to osd services will show up in the OSDSPEC PREVIEWS table changes to all other services will show up in the SERVICESPEC PREVIEWS table this is because osds are treated a little bit differently when generating the dry run preview. based on your description of your sequence on events this all looks correct. every time you run 'ceph orch apply osd -i spec.yaml --dry-run' your preview shows you the osds that would be added to the cluster.
i feel that the current table names are sufficient and self explanatory. as for explaining what the 2 tables are in more detail that is probably something that should be handled in the documentation not a print out from the command. the current warning printed out from the command is only there to REALLY make sure the user understands a dry runs are not a guarantee that apply the same spec later will result in the predicted outcome
Since the problem described in this bug report should be resolved in a recent advisory, it has been closed with a resolution of ERRATA. For information on the advisory (Red Hat Ceph Storage 5.0 bug fix and enhancement), and where to find the updated files, follow the link below. If the solution does not work for you, open a new bug report. https://access.redhat.com/errata/RHBA-2021:3294