Saying each writer has a unique voice does not mean that each writer has a radically different style from anyone else. In academic writing, voice comes down to small habits and personal preferences. Think about it this way: if all the students in your class were told to explain a complex concept, none of them would do it in the same way. Each one would use different language and syntax to describe the concept, and as each student makes individual choices in language and syntax over a period of time, their readers will eventually associate those choices with particular writers — their unique writing accumulates to create an authorial voice.