I am not mixing up the 2

First row, for https://livingwage.mit.edu/counties/06075

    | 1 adult                                        | 2 adults (1 working)                          |
    | 0 Children | 1 Child | 2 Children | 3 Children | 0 Children |1 Child | 2 Children | 3 Children |
    | $29.31     | $61.37  | $83.72     | $107.95    | $41.83     | $50.47 | $54.77     | $63.97     |

    1 adult + 0 children  = $29.31
    2 adults + 0 children = $41.83

The only way these numbers make sense if if you assume one income. Then

    1 adult + 3 kids = $107.95
    2 adults + 3 kids = $63.97

Given the first example was one income, this 2nd one makes no sense. 5 people cost more than 4. These numbers are wrong.

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.