If there's an undulation in one wall (and not the other) the cam can take advantage of this and reduce the camming angle, making it more solid.
I always thought (but never got around to testing) that a poorly lubed cam is very bad in slick rock, not only because you get less effective transfer of down-to-outwards forces, but it can't react quickly to a sudden load such as giving it a tug.
If a cam stem isn't aligned with the direction of load, you are giving up some of the available holding power, even more so if it's not lubed.
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