My hunch is that the owners are from Hyderabad. Chicago has a large Hyderabadi-Muslim population. Many dishes on the menu skew Hyderbadi - baigan bartha, mirch ka salan (traditionally served with biryani), bitter gourd, etc. etc. The inclusion of some of the more squarely northern Indian dishes seems to me a concession to the expectations of a large and diverse expat and western crowd that is more familiar with northern Indian food. Ghareeb Nawaz is for everyone, after all. Hell, they even have kapsa, a dish common in the Arabian Peninsula, which would be familiar to many south Asians vis-a-vis having worked in the region or having family who have. But I think they're Hyderbadi. Judge a place by its most unique offerings, not the general ones.
The people who now populate Pakistan are a mix of people who lived in that country's boundaries, as well as immigrants from all over India who came during/after partition. A small Hindu minority even remains. As do Christians. Pakistan also hosts hundreds of thousands of refugees from Afghanistan. While Pakistan's cuisine is certainly similar to northern Indian Islamic/Mughal cuisine, it has absorbed many other influences. For example, I believe frontier chicken is a reference to Pakistan's frontier provinces bordering Afghanistan. Nihari seems to be abundant on both sides of the border, as does Paya. I'll venture and say that garam masala is particularly typical in Pakistani cuisine. You'll find different spicing patterns in south India (where Hyderabad is located) and east India.
It's not fair to speak about Indian food as if the "real" thing is dosas, utthapam, vegetarian stuff, etc. More Muslims live in India than in Pakistan, and they live in all areas of the country.
I'm not sure this answers any questions. It's a culturally complex region, which is a wonderful thing. But the point is that I think people tend to label Indian and Pakistani based on whether they perceive the restaurant to cater to Muslims or Hindus. Just because a restaurant serves beef brain curry and goat feet doesn't mean its not Indian. In that region, it would probably be easier to talk about restaurants on the basis of the regional cuisine they serve. Thus, you could have Sindhi food, Punjabi food, Keralan food, Mughal court cuisine, Gujarati, Jain, etc. etc. etc.
For the person looking for spicy food for the sake of spicy food - you are better of searching this forum and looking for well-loved restaurants. You'll have a great meal, and if you really wanna burn your bunghole you can ask for you dishes extra spicy and order some chopped chilis on the side. I think this is a better approach than to search out some "how spicy can you take it" gimmick dish. But that's my two cents, and what the hell do I know about eating in Chicago.
"By the fig, the olive..." Surat Al-Teen, Mecca 95:1"