The deliberate mistake is the fact that the upper disc of the left hand set technically should not have been blacked out as it was the one that read to the continuation of the platform line, the lower disc orginally read to the crossover road.
The rule is that the top disc of a 'stacked' set read to the leftmost route and the bottom to the righthand.
So, originally on the left hand pair the TOP was STRAIGHT ON and the lower was over the crossover. For the right hand pair on the other hand the TOP disc was a route over the crossover and the LOWER was STRAIGHT ON.
regards
John