Basically in the first few months of destiny, when you bought an exotic engram from xur you had the chance of getting ascendent shards or energy from them instead of exotics. And same with legendarys, you could open one up and get a blue. I got my very first exotic (ice breaker) from a legendary engram from this time period. I was considered extremely lucky because getting legendarys and exotics from legendary engrams was more rare then getting exotics from legendary engrams now a days. Hope this clears it up.