It's an artifact of the projection also being used to slide off walls smoothly, and then just reusing that idea - which could be a accident, or an "accident" (choosing by what feels better).

Goldsrc surfing was most exploitable in the form of "wall-running" by strafing against rounded surfaces(e.g. the cylindrical sewer pipe interiors in cs_siege and cs_militia, I think, both did this). Same bug, different mechanism, easier to control.