Hello mingming82
Since we are all volunteers running this help forum, a hello and a thank you is always appreciated 😃.
The 13.50 is the half fare tariff for this route. Full fare would cost 27.00.
The SBB webshop always defaults to the half fare tariff as the majority of the Swiss residents own the Half Fare Card. Therefore, when not logged in, the lowest possible price displayed includes half fare.
As you are logged in and correctly made the Half Fare setting as described here, the price is not different no matter if you are logged in or not.
Kind regards,
Roland