Lee,
Memorial Day weekend might be the single best time of the year to try that restaurant that's tough to get into (with the exception of very touristy River North places), since everyone who lives in Chicago seems to leave, replaced for the weekend by extremely perplexed driving touristst from a heartland triangle defined approximately by PA, NE and TN. That goes for shopping too. I actually parked near the entrances of Sam's and Costco, among other places, in the middle of the day on Saturday and also didn't have to stand in line. I'd chalk up your experience to this phenomenon, which must leave the tourists with a skewed impression.
Also, I know from family experience that Puerto Ricans and Cubans often eat very late, like Spain late. Same for most South Americans, but seemingly less so for Mexican families. You might notice people eating at spots like Cafe 28 or Tango Sur way after most restaurants are empty.