Not sure if this is a bug or enhancement - 

When the GIS plugin is installed in Postgres, it creates the spatial_ref_sys table within the user schema by default. This table seems to be a data dictionary table and not part of the user data. 
Currently, if a exclude list is not provided, it gets synced by default. In the postgres-example, the GIS module is installed, resulting in this system table being synced as part of the standard example demo setup.
I suggest to exclude this table by default within the Postgres connector's settings.