i think because standart amxx plugins using 1 or 2 afaik. so they will not mess, and developers did standart as 4.
and if you would read description for native, you will see again about -1.
PHP Code:
/**
* Sets format for hudmessage.
* Note - as of AMX Mod X 1.61, setting the channel to -1
* will automatically choose the next available HUD channel for a player.
*/
native set_hudmessage(red=200, green=100, blue=0, Float:x=-1.0, Float:y=0.35, effects=0, Float:fxtime=6.0, Float:holdtime=12.0, Float:fadeintime=0.1, Float:fadeouttime=0.2,channel=4);
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