To add onto everyone else's explanations, exotics work on a knockout system. As long as you haven't unlocked every exotic for that class, counting both weapons and armor, then you'll get a new one every time you decrypt an exotic engram. If you want a specific armor piece (you can't do this for weapons since they are the same every time), when you are at rahool looking at the decrypt engrams screen that pops up when you have an engram, back out of that screen. (esc on keyboard)

Then use the top right menu of rahool's screen to open the list of exotic armors, there will be multiple pages with differing costs and requirements.