There's a deal of conspiracy theories about item drops floating around.
Some believe that Blizzard have a 'luck timer' that determines how often, and how voluminous legendary drops will be for you.
Blizzard themselves have apparently stated that if a player is bereft of legendaries for a set time, the game will force a drop of a legendary.
Within the clan, many members prefer to solo as they feel they get more legendaries this way, as when they group, they get none/few.
Myself, I have found that I played a proper session on Sunday (about 10hrs) doing rift groups and cache groups all day. The drop rate was typically 3-4 legendaries every 2nd rift. With one rift giving nothing. This was on T4-T6.
T1-3 my drop rate is about 1-2 every 2nd rift.
My first rift of the day is usually fruitless.
I am quite sick of getting the same old items though, despite playing on even harder difficulties, getting 2 more Tyrael's Might instead of 1 is hardly awesome. I'm about Paragon 220 now, and still have had no Magefist or Cindercoat drop yet!
Can't even begin to consider what the wait for a Calamity, Kridershot or other super rares for other chars will be like...