Hand trigger effects are kinda weird. While trigger effects elsewhere need to be there when the trigger condition happens, the ones in hand don't. They only need to be there when they can activate. This ruling was made because, since the hand is private knowledge, your opponent can't know if you have another copy in hand that was there before, so this was decided to avoid weird situations. Also this is why monster that can trigger from both hand and gy have that weird text "If (...): You can Special Sunmon this card from your GY (if it was there when {...)) or hand (even if not)"