Run the following code from the debug window during a vanilla mission.

for "_i" from 1 to 10 do
{
	radioChannelCreate [[1.0, 1.0, 1.0, 1.0], format ["Channel %1", _i], "", []];
};

for "_i" from 1 to 10 do
{
    setCurrentChannel 3;

    _i radioChannelAdd [player];
    setCurrentChannel (_i + 5);
    systemchat str currentChannel;

    if (_i != 10) then { _i radioChannelRemove [player] };
};

The chat window will show

6
7
8
9
10
11
12
13
14
3

Indicating that ARMA will allow a script to select custom channels 1 through 9 (reported as 6 through 14) but not 10. The 3 on the end indicates that selecting channel 15 by script failed.

If you then manually cycle through the available channels after the script finishes, you'll find that your player has access to Channel 10. The channel is there and accessible, but setCurrentChannel will not work with it.