The reason isn't clear, but the unofficial notation of 'Keihan Uji' was used until recently (in the past, it was customary to use unofficial station names because it was sometimes necessary to use notations on designated round-trip tickets and also to differentiate this station from the JR station with the same name in transportation connecting with JR).
なぜか正式名称ではない「京阪宇治」という表記が近年まで用いられていた（補足：かつては周遊指定地接続駅の表記と合わせたり国鉄との連絡運輸の際に国鉄駅と区別するといった理由で、正式名称にはない会社名を付けて表記することが行われていた。