Before we checked in at the GDT, we've been reading that on the boards too. Now that we're here, we do understand why: most of the times, guests on the 15th floor will be CL, maybe not because of a policy of the resort, but because the layout of this floor is very peculiar.
Try to picture yourself the 15th floor and it will immediately make sense: at the standard view side, there is the Chronos Club that is taking a whole bunch of space (9 windows). Adjacent to it, three rooms and, further down, at the location where the tower “bends”, the two “triangle-shaped” deluxe suite. One of these suites communicates with a room. Then, on the north side, there is one of the two presidential suite, that takes 5 windows and further away, one room. On the south side, next to the deluxe suite, there are seven rooms.
On the water view side, there are fewer windows and rooms (because of the way the tower is shaped). In the center, opposite to the Chronos Club, you have the Alcazar Suite, taking up 5 windows and two L-shapes deluxe suites, one communicating with a room (3 windows for each suites). Then, after the tower bends, 5 rooms on each side.
To sum it up, the 15th floor has 6 suites (all Club-Level), that communicate with 4 rooms.
It also has 9 standard view rooms (probably Club-Level all the time, since this is the bookable category for CL) and 10 water view rooms, which is not a lot. Last, when the Alcazar is booked, most probably their guests will also have reservations for some other rooms, which Disney is going to locate on the same floor.