In a TAS, the game is being played by the computer using a series of pre-programmed inputs. The fun in TASing is creating the most optimized set of inputs that completes the game in the shortest amount of time. There are often precise one-frame tricks or inputs that would not be possible for a human to perform but can be done easily when you can fully control what happens on each individual frame of gameplay.
Mixing TAS and non-TAS runs doesn't make much sense as humans would never be able to compete.