The following census results are read from the original documents. There is no mention of being born in Illinois on any of them.
1900 Census: 261 1st St. Hoboken, Hudson, New Jersey
https://familysearch.org/pal:/MM9.3.1/TH-267-11870-11880-97?cc=1325221(and next page for the children)
Spence, John, b. Apr 1868, 32, married 5 years, b. IRE, immigrated 1888, 12 yrs in US,
brakeman (R.R.)
Mary, b. Aug 1871, 28, married 5 years, mother of 2 children, 2 children living, b. IRE,
immigrated 1893, 7 years in US
John A, b. Apr 1896, 4, b. New Jersey
Maggie, b. Aug 1898, 1, b. New Jersey
1910 Census: 58 1st St., Hoboken, Hudson, New Jersey
Spencer, John, 42, marriage 1, 15 years, b. IRE, immigrated 1888, naturalized, brakeman,
steam railroad
Mary, wife, 37, marriage 1, 15 years, mother of 5 children, 2 children living, b. IRE,
immigrated 1893
Margaret, daughter, 11, b. New Jersey
Alexander, son, 7, b. New Jersey
Alexander, boarder, 34 b. IRE, immigrated 1905, naturalized, watchman, railroad co.
1920 Census: 261 1st St. Hoboken, Hudson, New Jersey
Spence, John, rents home, 52, immigrated 1889, naturalized 1896, b. IRE, brakeman, railroad
Mary, 49, immigrated 1891, naturalized 1896, b. IRE
Margaret, 21, b. New Jersey. stenographer, ? office
Lillian, 17, b. New Jersey
1930 Census: 261 1st St. Hoboken, Hudson, New Jersey
Spence, John 52, married 1st at age 32, b. IRE, immigrated 1888, alien, brakeman, railroad,
not a veteran
Mary, 50, married 1st at age 30, b. IRE, immigrated 1893, alien
Margaret, 16, b. New Jersey
Adeline, 14, b. New Jersey
Sophie. 12, b. New Jersey
John, 10, b. New Jersey
It looks like the ages on this enumeration are probably all off.
1940 Census: 261 1st St. Hoboken, Hudson, Hew Jersey
https://familysearch.org/pal:/MM9.3.1/TH-1971-27895-2908-63?cc=2000219Spence, John, head, 76, b. Eire-Ireland, lived same place in 1935, switchman, steam railroad
Mary, 69, b. N. Ireland, lived same place in 1935
Margaret, 41, b. New Jersey, lived same place in 1935