Can someone please explain to me how you manage to feed a Yasou as Jax and a Rammus

RANT AHEAD . A good five ten or so minutes into the game Rammus tries to gank the Yasou with Jax both are near full health and Yasou gets a double kill , this happens again . Now that hes fed he just stomps the game and destroys all the turrets and gets his entire team fed ( good job on it ). Which brings me back to my question how is this possible? They were really bad ( at least in my opinion anyways ) in their decision making skills and they both had twenty deaths between them , but I dont know.