Please correct me if I am wrong, only it has happened twice in two days now!
Just the same as all roundabouts you give way to traffic already on the roundabout coming from the right.
Just because it is a small roundabout it makes no difference to the rules.
Vehicles already on the roundabout have sort of right of way but you may proceed if you will not cause them to brake or swerve - when this rule is not followed confusion arises and the traffic flow is disrupted.
Sadly mini roundabouts have in many cases replaced a cross roads or T junction and people who have not read the highway code for years and/or have known the junction for years may tend to treat is as if it has not been changed - sadly these are the idiot drivers we all have to look out for as there are so so many out there.
The correct terminology is "priority" not right of way. Having said that, you are right in your understanding of the use of mini roundabouts. Drivers should NOT drive across the white circle, they should go round it. Percyqted missed out a word - The graveyard is full of people who were DEAD right!!
You understand the rules of roundabouts just fine. However, you seem not to have picked up a more fundamental rule of driving - always assume that every other driver is a moron who doesn't know the rules.
Yes the person on the right had the right of way but many people do not pay any attention to the policy.
The graveyard is full of people who were in the right.
You must learn to balance priority and prudence.