Well, i dont whant to create conspiracy, but for my experience there is "Day-time" for each faction/expansion kegs.
Example, almost all The time i play The game in cellphone After i go to bed, and almost The time i open kegs in that period.
The SC was The faction i have less cards, and i opened a lot of SC kegs in that time.
So i try to open kegs in The afternoon and got a lot of legendaries. (i made that test more than one time).
So my tip is, buy 20 kegs for a faction /expansion, If you dont get to many legendaries, buy more 20 kegs for another faction/expansion and see whats happens