99% of the time they are Hacked Pokemon. For instance, PokeFlexii sent me loads of shiny Pokemon via GTS, but I checked the Ivs and they were all perfect! To add to that, they all had egg moves. However when I deleted Dragon Dance on my Shiny Salamence, I couldn't get it back from the Move Relearner! All of the shiny Legendaries I got had perfect Ivs too, so that is an EXTREMELY rare chance of getting a Shiny Legendary with a decent nature with perfect Ivs! They post in their stream that they need (for instance) a level 1-10 MALE Grimer with the name insert YouTuber name here then they know who to trade. 90% of the time they will trade you.
They are hacked.