I belive the process of enchanting is totally randomic:
You are enchanting your weapon, so you send a code to server(database) who speak your weapon is +X , and then , server send a randomic number(like 1 or 0, broke or enchant) based on the enchant +X to client. If you can exploit server randomic number(It's possible, but if anyone discover i belive he will no speak to anyone) or maybe one program who send to server you are enchanting a +3 wep and not a +4/5/6/...
And other thing, I belive the randomic number who is generate by server are pure probability and then : First enchant: 50% chance to be succes(1/2) Second enchant: 30% (1/3) and blablabla for example, so the chance to enchant to the second enchant is 1/2 X 1/3 = 16%
So there's some bug or trick to incrase chance to enchant to +25 on a server who the safe enchant is +3...
Like L2Gold, who have a halisha +19, are he crazy to enchant a weapon +18 to +19 without know if it will broke or not, or he know it has a good chance to enchant to +19, he scammed 8+ halishas 16, he is VERY rich or he is GM friend...
No one belived it was some dupe bug and then: it's appeared... if there's a locked door there's a key to open it