The Campy head in your photo wedges into an acute angle. Gravity, pulling the pump down, would wedge the pump in TIGHTER, which keeps it in place. Try to remove the pump by just pressing it down at the Campy head end. You probably can't do it. It's self-securing.
That same Campy head, pressed into an obtuse angle may hold steady with the bike parked. Bounce over a bump, as the pump slips down, it becomes LOOSER, and eventually falls out. You'd need a strap to keep it secure
BL in EC