Flow not... flowing as expected

I’m trying to use Flow to apply a tree-like pipe structure to a curved surface. It’s not working as expected – it’s approximately right, but does not match all of the curvature.

…I am expecting the branches at the bottom to align with the surface, but instead they aren’t rotating as much as the surface does. Feels like one of those “missing something fundamental” moments, but I’m not sure what. Ideas?

branches-debug.gh (49.0 KB)

Flowing the bounding box makes it clear that the curve is being honored, but the rotation is not. Probably means I’m misunderstanding flow. Is there a better way to do this? Goal is to have box laying along whole side of surface.