There is more luck involved that I expected.
I got this nice sword from a chest but the game crashed and I lost my item. I really wanted it again. When I reloaded and opened the chest a second time it gives me this killer legendary Axe. well i dont use axes, so I figure I could re-load to see what the next legendary item would be. Nope, that chest never gave me anything more than oranges no matter how many times I reloaded from that point. I should have taken the axe. Or at least SAVED to a different location so it was an option.
Its a lot of luck involved, I thought a chest that gave you something nice would always do so, but I found out differently.