¿Por que 'separado' se escribe todo junto y 'todo junto' se escribe separado?

A Wholesome Bowl of 41 Funny Feline Treats for Cool Cats With a Whiskery Sense of Humor 1

41 Hissterical Tweets About Fluffy Felines That Are Pure Catnip for the Hooman Soul 2

32 of the Funniest Felines on the Internet and Meowrvelous Memes to Claw Your Way to a Purrfectly Wonderful Weekend 3

Top 20 Cat Memes of The Week - Cheezburger Users Edition #344 4

Midweek Meowgical Moments With 24 Hissterical Cat Memes For The Purrfect Pick-Me-Up 5

37 Funny Feline Memes to Whisker You Away to a Purrfectly Pleasurable Weekend 6

Might as well... 1

32 of the Funniest Felines on the Internet and Meowrvelous Memes to Claw Your Way to a Purrfectly Wonderful Weekend 2

He's innocent 3

Every time 4

Prank call 5

Some day, some way 6