Without checking, I don't know the answer to your question. My guess, though, would be that it's to do with what routes are permitted by the Routeing Guide for a Worthing-Chatham ticket. It probably means that via Tonbridge is not a permitted route for that ticket, with the result that you have to split the journey into parts and buy separate tickets for each part if you want to take that route.
For example, you can get a Worthing-Cuxton offpeak day return valid via Tonbridge for £20.50, and a Cuxton-Chatham offpeak day return is £3.70.