Enchanting Issue

So whenever you leave your world, you will always get the same enchant option. I find that to be a pro and con. I cannot leave my world freely and ensuring that the enchant I want is there. I have to be able to enchant the item that I want and then leave.

Example: Fortune III option. Enchant it and you get a new enchant, fine. Then you leave the world to do something else, come back and you still get the same fortune III option.