Could be that the flight starts somewhere else, they can check-in earlier. Depending on how many seats are on the plane, C can go up to 55 (175 total seats).
If the flight is coming from another city and making a stop at your city before going on to MCO all those people coming from the first city had their 24 hr marks hours before you.
And it some time is luck of the draw. Last year I bought a ticket to Disney 6 hours before the flight left. I checked in fully expecting to get the C board position. I was shocked I got an A position.