Originally Posted by Alan Shore

Now I'm with you. I believe the reason is that, to count as a "Reserve Available," a pilot must be on call for the entire day. A pilot who begins short call at noon is not on call the entire day, as he is at rest 0000-1200. Before he was assigned that short call, he was counted in Reserves Available, but once the short call and the requisite prior 12 hours' rest was put on his line, he was not.

OTOH, a pilot who carries a short call into a day from the previous day does count in that day's Reserves Available because he is on call, i.e., not at rest, for the entire day.