I think you are onto something but it may not be solely account bound. I think every character you make has different rolls. For example on my titan i have found 3 or four icebreakers but have never seen it on my warlock or hunter but on my warlock ive seen 3 no land beyonds. I think certain weapons and armor are harder to drop based off of the characters created. And obviously its not impossible to get them all even on one character.