Hiruzen is said to know every jutsu in the leaf. Firstly it doesn't mean he can use those jutsus. There is a difference between knowing and being able to use. Secondly, like you've said, Hiruzen is restricted to Konoha's jutsus, when Kakashi copied jutsus from even other countries and villages all over the missions he completed, so his knowledge of jutsus has a greater scale. Furthermore, it's not said that Kakashi copied exactly thousand jutsus, but over thousand. Even after Zabuza talked about his nickname, he even copied Zabuza's ninjutsus during the same fight, he copied Kisame's suiton later, so obviously, when you have thousand jutsus and completing missions and copying jutsus every time, you'll have more. It's logical. Like i've said, Gai said Kakashi is the number1 technician in the entire leaf and Hiruzen was still alive