In Illinois, presidential primary petitions need their own petition of 3,000 signatures. And if they want to elect delegates, separate petitions are needed in each U.S. House district. According to this story, Republican presidential candidate John Kasich failed to file enough valid signatures in six of the eighteen districts.
Also, an individual named William K. Graham of Glen Ellyn has filed a challenge to the Illinois Marco Rubio petition.
How would Politico know the numbers?
There must be some Politico reporter who lives in Chicago and went to the State Board of Elections office to look at them.