Why do some disaccharides give positive Benedict's tests while other disaccharides give negative tests?

Basically, Benedict's test identifies the existence of aldehydes and alpha-hydroxy-ketones, also by hemiacetal, as well as those that take place in specific ketoses. Therefore, it is an alpha-hydroxy-ketone even if the ketose fructose is not strictly a reducing sugar, and provides a positive test since it is transformed into the mannose and aldoses glucose by the base inside the reagent.