Why do some people find roundabouts so difficult?

People often turn left from the right hand lane so we're watching the truck.
What do B15 JNH and FE03 SXC do? Overtake on the right and then cut in and start to pass it on the left. With FE03 SXC having to make brake hard to avoid hitting both the truck and his 'gay driving' friend. 'gay driving' is the term coined when two drivers (usually male) are in separate cars but clearly cannot bare being more than 6 inches apart.
This doesn't look quite so impressive in the clip as it did on the day as the camera has a wide angle lens making distances look longer.
And note how rapidly, in the 30mph limit, the two of them disappear...