Originally Posted by rehoult

Based on the comment "after putting in the numbers", I expect that he had selected "show only upgradeable fares". In which case it is likely that they've put on B fares at a discount, but U fares are still running at full price. No reasonable person would buy that U over the B, but by ticking the box you are asking the system to display the M/U fares, its then up to you to make the smart decision.