And, hence, lies, the oxymoron in the term itself. A mixture is composed of different items that somehow come together yet can simultaneously maintain their individual identities. An air conditioner continuously mixes up the humid, warm air with the cooler drier air. Because it is a mixture of air, literally. Cold and warm airs constantly get mixed to form a uniform cloud.
Different clouds get mixed together to form storms, and so on.
You get the idea. Torah is also a mixture of different characters, ideas, and concepts. Yet they are put together in one Torah. It began with a small group of freinds who were interested in learning Torah together in small groups. Hence, he named it chevruta which means a group of friends.