Either it's a random process w/ variables or it's so straightforward a few possible illegals are ruining your chances. Gotta pick one. ;-)
Well, I have not researched every colleges admission procedures but I know UT-Austin's pretty well and it's a little bit of both of those.
It's straight forward in that 50% of it is an formula based on academics that gives you a score. The other 50% is the PAI, (Personal Achievement Index) which is a subjective score based on your essays, extracurriculars, and personal background. These are combined to form an hypothesized freshman year GPA. Then a computer lines all of the scores up, and they say they need to admit however many applicants based on their expected yield. The computer draws the line at the number and everyone above that above that line gets accepted and everyone below it gets denied.
So it's straightfoward in that you could be the first person on the wrong side of that line, and it be because an illegal immigrant was on the good side of the line.
The random part is, you can't control how strong the other applicants are. So an application that might have gotten declined one year could be accepted the next year.
Frankly I don't quite understand how any "illegal alien" could complete the mountain of paperwork and data necessary for college admission (medical, financial, transcripts, SSN etc.) without already being on the government's radar..
Well they are, but it's the state government that runs higher education. And we all know what happens if the state tries to enforce federal laws. ;-)