There are times when I try to Check in and in the case of Shuttle flights grab an assigned seat and also get the 24hr remark even though I do it inside of the 24hr period. The glitch that I've noticed on these occasions is that I am able to check in at 24hrs but can't get my seat assignments until 24 hrs on the clock in Phoenix.
For example, if I had a 6pm flight out of LGA and tried to check in at 630pm EST I get notice that seats aren't available until within 24hrs of the flight. I would continue to get this until 9:00 pm EST or 6PM Mountain time.