They conveniently abused any rules they can find in Quran. The famous one is Taqiya, the rule to trick your enemy with deception to save ones life... in events of war. They can say it is okay to eat pork and drinks alcohol when situation permits, like blending in with your enemy. So they declared war, which never reciprocated and no Grand Caliph existed. So they created the Caliphate. Fasting for only 3 days? Sure, since they have the Grand Caliph... no need to consult the Imams or Hadith, let alone settle on council meeting.