For example, I consider myself to be a mediocre illustrator, but a decent comics artist (granted, with plenty to learn). I can't quite make that final leap to producing the perfect, dimensional, compositional, self-contained aesthetic world of a stand-alone illustration...but I feel quite comfortable in a sequential art setting, even if it's only a single page. When confronted with an illustration, I tend to immediately seek out a big cheap visual metaphor or device, whereas in comics I generally back off on the "clever" factor and let the characters or world or flow of time speak for themselves.
It's not particularly related to drafting skills - I'm using the same hands for both - but I have difficulty committing to an illustration in the same way that I have difficulty committing to a short story while writing prose.
Many people seem to be quite ambidextrous in their ability to illustrate versus storytell. And others lump the aesthetic goals of their primary medium over the fence, and produce well-drawn but somehow stilted-feeling examples of the opposite medium as a result.