i have heard that every kind of enchant has (or could have) different chances...
Game has different codes for dropped, spoiled, chest, mammon, olimpiad or quest enchats.. and some of them have different chances.. Blessed ones have less chance of success also...
On private servers, GM can tube up the percentages to its desires..
Also different kind of weapons had different chances.. 1 hand ones are more prone to be breaken.. and bow s too..