I sort of think of SSI as I do minimum wage; some states use the default, Federal amount, and other states give a higher amount. The Feds give the States a default amount of money, based on the numbers of people in their state who qualify for SSI and, as Rose says, some "richer" states add money or programs to that. SSI is qualified for the exact same way as SSDI is as far as the disabled part goes, the Federal test qualifies someone for either or both. That's how the Feds know how much money to give the state, it's their test and their default money, it's just not funded by Social Security.