When you set MAVIS to auto mode we hand off any sensing/auto adjustment to the camera hardware. If you are shooting at 60fps the shutter may default 1/100 due to the environment. For example, to prevent strobing from a light source the camera will profile the surrounding environment and lock on to the refresh rate of the lights. Even when shooting at 60fps a shutter speed that is a multiple of 50 might work best for the environment.