Simple. There are pilots that "know" they cannot make a 12 hr call out at certain times of the day. Heck you cannot sit in ATL and make a 12 hr call out in JFK, EWR, or LGA if they call at 2000 for a 0800 report.

We have also had pilots sitting SC and have expected not go get called, only to get that call and then not be able to make it. Some have even gotten their call and then told the scheduler they were getting on a three hr flight to the hub from their home.