Playing fast and loose with the timeline??

The original Mannon episode aired in 69. The last episode of the series was in 75. This movie premiered in 87. In this movie, they are saying that Mannon has been in prison for the last 12 years. But it's 18 years from 1969-1987. It's 12 years between the last episode in the series and this movie. I wonder why they did this? It couldn't have just been oversight.