Start9 supports both core and knots. There is a bug in the old version of StartOS where both Core and Knots share the same id resulting in confusion like you express here. More details and information on how to switch are listed as a guide in the Guides category.
This is fixed in the latest version of StartOS (0.4.0). StartOS 0.4.0 is currently in public beta so your system will not show it as an available upgrade. If you are interested in upgrading StartOS - see the upgrade guide for details.