Look at the childcare number in the breakdown table. 1 adult and 3 children has an estimated $71k/year childcare cost, while 2 adults and 3 children (1 working) has a $0/year childcare cost. So some things go up (transportation, healthcare, food), but others go down. Childcare going down by $71k pretty much entirely accounts for the difference you're questioning (~$34/hour difference just on that entry).

Also, two adults (assuming married) will pay lower taxes than one adult for the same income. That's another ~30k difference per year in the breakdown table for the 3 children case. If your tax burden is lower, you can afford a lower wage while bringing in the same net.

EDIT: Tax rates in the US are roughly half (except for high income earners, way beyond these living wage estimates would be relating to) when you're married versus single.

https://www.irs.gov/filing/federal-income-tax-rates-and-brac...

Check out the 22% bracket on that page, the range is doubled for married people filing joint versus single. That's a huge savings each year. Tax savings of two married people and any number of kids is a major contributor to why the living wage drops when someone gets married versus is single with the same number of kids.