Professional projectors use interchangeable lens and can be exchanged to provide for different screen sizes and projector throws.
Lens are expressed for example 0.8:1 and 1.2-1.8:1. Lets look at the different types and what they are used for, to keep it simple the numbers we will refer to in feet.
The first digit represents the distance of the screen to lens front, and the second digit represents the image width.
A short throw offset axis lens enables the projector to be mounted either at the top of the screen, or at the bottom and projects up or down as necessary, this can be useful for front project where it is required to have a short distance to the screen from lens, however is unpractical for a on axis to have the projector dead in the middle of the screen. The lens features some very clever optics with when viewed looks very odd but the lens and often mirror create a perfect image with little or no adjustment as long as it is aligned to the edge of the screen.