This is due to a "fix" for a Quake 2 movement bug that seems to apply to upward stepping:

		// this has been disabled so that you can't jump when you are stepping
		// up while already jumping (also known as the Quake2 double jump bug)
#if 0
		// LordHavoc: disabled this check so you can walk on monsters/players
		//if (PRVM_serveredictfloat(ent, solid) == SOLID_BSP)
		{
			//Con_Printf("onground\n");
			PRVM_serveredictfloat(ent, flags) =	(int)PRVM_serveredictfloat(ent, flags) | FL_ONGROUND;
			PRVM_serveredictedict(ent, groundentity) = PRVM_EDICT_TO_PROG(downtrace.ent);
		}
#endif