yeh i know its not right...i meant if you suddenly find yourself in the wrong lane, realising too late you need to be turning right, then it is safer to continue in the left and indicate right to let others know you are not turning off, rather than trying to get into the right lane, or wait for a gap and hold up traffic etc, only to have to come out of it again a few seconds later to exit...this is of course assuming this is only an avergae sized roundabout...if a very large one then you shoudl have time to get into the right lane
other cars should never predict what they think other drivers are going to do, and only be sure they are turning off when they see their wheels turn, so the indicator is a fair warning...
yes its a mistake, but they happen and it needs to be dealt with as safely as possible