Under Joomla!, the backup process runs inside Joomla! no matter if it's a backend backup, frontend backup or a CLI backup. That's because Joomla! provides a native CLI API.
On WordPress, however, the backup process runs inside WordPress only when there's a web server involved, i.e. not during a CLI backup. As a result we do not have any way of reporting the correct WordPress version in CLI backups.
We had three options:
A. Do not provide a CLI backup for WordPress. This doesn't really make much sense. People need the CLI backup far more than they need this variable.
B. Provide inconsistent values in the web-based backup modes (backend, frontend, JSON API) and CLI backups. The latter would have to return a fake version number. We think that'd qualify as a bug because it's so inconsistent and confusing for people.
C. Provide the version of the backup software itself. This is consistent with the way Solo works (where getting the version of the script being backed up is outright impossible). That's what we chose to implement.
Nicholas K. Dionysopoulos
Lead Developer and Director