As with most (if not all) offences there are different degrees of harm and culpability and so there are different ranges of punishments for the same offence depending on the culpability of the offender and the harm that they caused.
There is also the fact that an early guilty plea will lead to a reduction of one third in the sentence.
This woman didn't just post nude images of the OW on an escort site giving the husband's phone number as a contact number, she also attempted to cause harm to his business and also their daughter. The judge said:
“One of the most egregious and vindictive aspects of this case is that you took the time to draft a lengthy and vitriolic message to their daughter. You sought to drag her into your unhappy existence which was shameful.”
Then, instead of pleading guilty, she forced the victim and her husband to attend court and give evidence. It was only after that, that she changed her plea to guilty.
The judge gave her no credit for her guilty pleas because she indulged in the “ultimate act of brinkmanship” by forcing her victims to give evidence, something he said had “maximised their humiliation”.
He said: “You were arrested and interviewed and were utterly unapologetic. The lack of remorse you showed was staggering. Acting in the way you have is totally devoid of any moral compass whatsoever.”
She was sentenced for two counts consecutively, one for two years and one for one year. She will be released at the half way point (just like that person's son you mentioned) unless the new rules are still in place in which case she will be released early at the 40% point.
In contrast, if she had not caused much harm and not done any real planning then the lowest category of this offence has a starting point of a low level community order with a range of a conditional discharge up to a high level community order