No, it's not. It should regenerate the cache only if you change something in the Scenery Library, which includes also enabling/disabling areas, even without adding new ones.
Do you use some kind of 3rd party utility to manage the scenery.cfg file ?
If you are sure your Scenery Library hasn't changed between session, the only possible explanation is that one of your scenery files is corrupted, so GSX cannot complete the cache regeneration, so it will do it again on the next startup.
If you enable Logging in the Troubleshooting page of the GSX setting, you should find something in the %APPDATA%\Virtuali\Couatl.LOG file that will be generated on the next startup, that will probably tell you what went wrong when regenerating the cache.
