The point is that the system seems to be wrong - some people seem to get barely enough to live on through Universal Credit; others work full time in min wage jobs and get little if any support from the state. In that context, it doesn't seem right that someone's part time earnings should be topped up to a sum well above the national average wage, when there are other people who are receiving so much less
If the two families have the same number of children, no disabilities, and a working parent, the UC calculation will be exactly the same except for the housinge element. It's the massive variation of rents in different parts of the country that creates the disparity.
Here, the amount you can get for a 3-bed house is almost £1,200 a month, if you live in Merthyr Tydfil, it's £423 - nearly £800 a month less. So of two otherwise identical families, the one in this part of Sussex would get almost £800 a month more. But they'd need it, as even with £1,200 a month being added to the UC for rent, they'd be lucky if they could find a 3-bed house for less than £1,400.
I also think it's absurd that these regional variations aren't reflected in the benefit cap, which is the same everywhere except for London. Families in the south are likely to be much worse off because of the high rents, although I realise it is easier to get work.