7 1/2" radial arm miter saw

[pyro3256]

So I'm rebuilding this old saw I got for free. Not a restoration. Just getting it serviceable, so I can use it.
I tore it down and started to polish out the rusty "arm bars" sorry I don't know the correct terms.
Any way I noticed that the saw "carrier" only had 3 bushings in it and not 4.
I confirmed on sears parts exploded views that this is the way it came.
But why!? Does any one know the science or therory behind this? Thanks

 

[Outlawmws]

Simpler to adjust & cheaper to manufacture by doing two fixed and square and one for tension on the opposite side. Not better mind you, just cheaper.