Because in Half-Life we can add new weapons by adding their info to ItemInfo array:
PHP Code:
#define MAX_WEAPONS 32
ItemInfo CBasePlayerItem::ItemInfoArray[MAX_WEAPONS];
HL have only 15 weapons and so we have 16 empty positions in array to store information about our new weapons. In my module i'm using small hack with hlsdk function
PHP Code:
// Precaches the weapon and queues the weapon info for sending to clients
void UTIL_PrecacheOtherWeapon( const char *szClassname )
to insert info about new weapons to infoarray.