ISIS seems like it is largely a peer to peer terrorist organization with little if any central command, except for attacks in Iraq and Syria.
Thus, many of the attacks are not ordered by their leaders but by terrorists that have no connection with ISIS. However, central command will not hesitate to take responsibility.
Sort of an Open Franchise agreement. So long as you kill the ‘right’ people it’s okay. The Home Office just wants to know the total kill count of infidels. How you accomplish this goal, they don’t really care.