Projectors can be mounted in four very different ways: front projection, ceiling projection, rear projection and rear ceiling projection. The projector's factory default is front projection.
Each type of projection method requires the projector to manipulate the image differently. Ceiling mounting the projector requires the projector to be inverted, which in turn means that the image must also be inverted.
To successfully perform rear projection, the image must be reversed, and to perform rear ceiling projection, the image must be inverted and reversed.