How does this sentencing work?

[QuickQuack]

I guess this could go in NP&E so mods might move it there, but this caught my eye today:

https://www.bbc.co.uk/news/articles/cvgkj148p7no

TL:DR woman keeps another woman as a slave for 25 years and also severely abuses her during the entire period, but gets sentenced to only 13 years in prison herself, with probably only 2/3rd served behind bars; meaning that she held someone captive and tortured them for 25 years but she's only going to be held captive herself for less than 9. Where's the justice in that? In a case like this, surely the prison time should be a multiple of the time you held your victim captive, not a fraction of it!

I know, I know, prison isn't/shouldn't be for revenge, retribution or purely for punishment, and should be for rehabilitation as well, but surely there must some element of punishment to fit the crime in addition to the rehabilitation? Am I unreasonable to think that this feels negligently inadequate given the severity of the offence?

[sixor8]

She still got a stiffer sentence than some get for manslaughter. Even killing by dangerous driving gets less. The perpetrator wasn't the full ticket shall we say, not that it's an excuse.

[Glassman]

Mother of 10.