With vanilla snow layers, you can add one more layer to the pile when targeting an adjacent block where appropriate; not the case for TFC charcoal.

iirc, this is done to facilitate charcoal insertion in bloomeries (not misclick add another charcoal layer when you click in walls).

Vanilla snow is also weirdly inconsistent in this respect. A single snow layer has no bounding box, but all other sizes do. You can place up to two layers of snow by right clicking on the side of a block, but no more than the first two. Charcoal isn't snow, nor is it remotely implemented in the same way, so I see no reason why it should act exactly the same.