I read somewhere that the law is applied judgementally because animated images, and generic porn searches (via image search functionalities in search engines) all count as ‘downloaded’ images (even if they’re just clicked once and nobody downloads them) and that it’s sometimes possible for people looking at normal porn to accidentally download child porn as Google etc are shit at policing their searches.
^^ this
plus if you look at the actual law an 'indecent image' might be a cartoon / drawing. It might also be a picture of a child on a beach. A 17 year old in a bikini.
If you got the Beano as a kid like I did then some of the images when a child was being caned may now fall into the 'most serious category'.
Add to that 'bulk downloading' where someone gets a file from a website they don't see everything in it until they open individual images, even if someone takes a look and then deletes it immediately, and by it I mean the entire file it is still in the 'possession' of the downloader.
There has also been some research to show that offenders offered reeducation are more likely to not offend in future.
Finally I think some judges are reluctant to jail after a guilty plea as the defendant isn't actually given the images, they are told how many images have been found and what category they are, but that can be quite subjective. If it goes to trial then there is more argument.
Judges tend to sentence on what the 'general picture' of the offender's interest is, a bulk download of porn which has a small number of images is looked upon differently to a person who has downloaded fewer but specific pictures.
The Game of Thrones books have a lot of sex in them, including one of a 13 year old with her new husband, that is legal. The TV show has depicted the same scene but the actors are older and that is legal. If you create a pencil sketch of the same scene it is illegal.