One thing you must understand is that, when he first started writing, he never expected to be world famous. He saw his books, not as mémoires, but as novels: collections of short stories about a fictitious vet called James Herriot, who just happened to have a lot in common with Alf Wight, and who was married to someone unlike his real wife, but possibly modeled on his first girlfriend. As for the stories themselves, they were fiction based on fact, inspired by his own experiences and those of other vets, plus anecdotes which did the rounds of the profession, and which the members considered believable. But some true (?) stories were just too bizarre to be included. Take, for example, this tale recounted by his biographer.
Alderton [who played Herriot in the second movie] remembered particularly one vivid anecdote. Alf told him about his earliest days in the Thirsk practice [in the 1940s, when things were a bit primitive] that was so outrageous it could not be used in the film. 'Alf said he had anaesthetised this cat with chloroform and cotton wool inside a jam jar but the cat had a heart attack and died. He said that he'd learned at veterinary college that one way of kick-starting a heart is actually to swing the cat quickly by its tail - it sounds ridiculous but it can sometimes just shake the cat and start the heart again. Alf said "I went outside and swung this cat round by the tail and unfortunately I let go and this cat disappeared over the garden wall and I didn't know where it had gone and couldn't find it anywhere." A week later this guy who was living three doors down came in to see him and said "Mr Wight, I was in the garden the other day, reading the paper, and this cat jumped at me from the top of the tree and hit me on the back of the neck and killed itself." And Alf said "Sometimes they do." But we never could use that story.'Well, the story is second hand, but I want to believe it.
[Graham Lord (1997), James Herriot, the life of country vet, Headline]