Michigan and Illinois's population growth of the 1990's vs other recent decades:

[minionofmidas]

May indicate an undercount in 1990. I remember that when the 2000 census came out, the overall increase and that in several states was surprisingly high.

The 1990 census' flawedness (by high first world regular censustaking country standards) is notorious, and was known well before 2000 and recognized by the Bureau itself.

Wouldn't go so far as to try and explain this issue on that alone, though.