What you have to consider in that "creep up" is Kenneth Clarke became Justice minister and a lot of those short terms turned into community service! So of course the average term crept up when a lot of the lower terms were removed from actual custodial sentences! You can't just draw straight lines like that without deep diving as to what made that change.
It has been reported that this guy went back to reception after being released and was told again he was free! It would be pretty hard to find this was 1 person guilty. Sounds like multiple people just acting on whatever the paperwork said!
If the guy wasn't a deviant this would be a good episode of Porridge. Basically released against his own will and then sent away whilst trying to hand himself back in, sparking a national manhunt for a man who had no intention of escaping and by all accounts was wandering around London completely confused by the situation.
Reminds me of escapee, terrifying "super spy" and comedian Daniel Kaliffe. "I ****ed myself up" by clinging under a food truck to escape, "I dunno how immigrants do it!" Khaliffe wanted to "exude a sense of professionalism" Bond style by bringing a notebook and a change of clothes, only to be apprehended riding a bike along a nearby canal days later. "I never, ever thought it was a crime to "leave prison", I just thought it would be like, oh they catch you and put you back in... a case where I'd be on the news, I didn't think little Susie trying to go to Ibiza is gonna get her fricking plane cancelled and **** like that!"