When you install script, dialog is open and version is always <unknown>.
If you reinstall script, current version is displayed correctly.
If it’s a raw script file, the code would need to be run to work out the version (which could come from something more complex than a string constant), and running the code before it’s installed could cause problems.
Scripts shared with the new sharing function will have metadata added that lets their version be shown before installation. (If I remember correctly. Not at my PC as I type this.)