FAQ: Transport queue too short
When a transport is waiting for goods in a city, its position in the transport queue is only updated at 10 minute intervals. At these 10 minute 'ticks', new transports will arrive in the city, and those that have been filled to their minimum load will depart. In between ticks, though, players may manually order transports to leave the city, with cargo or without.
When a player orders a transport to leave the city early, the queue length will be reduced by one to reflect the fact that the transport is gone. However, the queue position of other transports in the city will not be updated until the next tick. If a player looks at one of these other transports during that time, its queue position may be higher than the overall length of the queue.
This will correct itself automatically at the next tick, when the queue positions are recalculated.