On the second photo you'll see the left lifejacket has what looks like 2 pieces of pipe on it. If you are a yachtie you clip your lifeline onto them and then when you fall over the side the yacht drags you along in the water. The one on the right you would need a seperate harness or you go for a swim.
On a rib you don't normally use a harness. So you just have two bits of pipe on your front for no reason.
Other note though - the harness one has a spray hood. So if you do go for a swim you have a plastic bag to pull over your head and keep the spray off a bit....