I don’t think there is one answer, nor do I think there has been much research on the subject, but here are a couple of reasons. When you draw a circle or a really straight line, you probably anchor your wrist and forearm to the desk and move your fingers and wrist. This removes a couple degrees of freedom. It’s harder to get a good anchor with a computer mouse. Also computers change the dpi (basically how far the cursor moves for every x distance the mouse moves) based on the speed of the mouse. Essentially the cursor moves more per inch the mouse moves when the mouse moves quickly than it does when the mouse moves slowly. This makes mouse speed a factor in cursor distance and adds another degree of freedom. With a pencil, if you move the tip an inch at the speed of light or the the speed of a snail, you still end up with an inch.
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