An Illinois resident cannot legally buy a gun in another state without shipping it to a FFL in Illinois and getting a background check done there before taking possession.
So the argument that Chicago residents can skirt gun control laws by crossing state lines is false.
"Anyone may arrange his affairs so that his taxes shall be as low as possible; he is not bound to choose that pattern which best pays the treasury. There is not even a patriotic duty to increase one's taxes.
Over and over again the Courts have said that there is nothing sinister in so arranging affairs as to keep taxes as low as possible. Everyone does it, rich and poor alike and all do right, for nobody owes any public duty to pay more than the law demands."
-Judge Learned Hand, Helvering v. Gregory, 69 F.2d 809, 810 (2d Cir. 1934), aff'd, 293 U.S. 465 (1935)
I'll guess the data is skewed because University of Phoenix caters to people who are already working. Work experience + degree is better than just a degree.