This is why I was surprised to see that it is considered less formal across the board according to the "rules." I suppose shell does have a heavier look to it, which may be part of the reason, since greater levels of formality often lean towards a less heavy look. "Delicate" doesn't seem like the right word to describe greater levels of formality for men's clothing, but some other word to that nature is what I am trying to say. I'm sort of grasping at straws here though. I agree that it depends on the particular shell... the older AE shell shoes certainly do seem more casual due to their opaque look, which almost reminds me of caramel or toffee candy. However, Alden, other manufacturers, and the new AE shell finish frequently show a level of shine and smoothness that should compete with a formal calf any day of the week.
Something else to consider between shell vs. calf. The huge rolls in some shell, e.g., plain-toe shoes, may make it look more casual in comparison to its brothers in calf, Some shell may crease very little too though.