average life sentence?

"When Longford first meets Hindley he tells her the average length of a life sentence. When Longford takes his wife (I guess around 1980) to visit Hindley she's been in prison fourteen years. On the train home Longford tells his wife that the average length of a life sentence is twelve years.

What year did he first meet Hindley and what did he tell her was the average length of a life sentence? Was this a deliberate comment by the film-makers about how a life sentence means less than it used to? "

When he originally spoke to Myra he told her that the average length of a life sentence (by which I took to mean the tariff indicated by the sentencing judge) was 21 years and that she could apply for parole after one third of that had passed, i.e. 7 years.

When he was speaking to his wife on the train I presumed that the time frame he was talking about i.e. 12 years was the average length of time served by a person sentenced to life imprisonment, which is subtley different to what he said to Myra.

Luc