You can't be certain if those "claiming credit" actually did it or not
Several years ago, a Lauda Air 767 airliner came apart in flight, killing everyone on board. I recall that at least one terrorist group claimed they'd brought down the plane with a bomb. In fact, it was downed by a freak accident where one engine's thrust reverser activated in flight. It was purely a mechanical malfunction.
It's possible many terrorist groups and wanna be's claim credit for these attacks. They want to be able to claim to potential recruits that "Yeah, we did that!" Just to be sure, we should take them at their word and whack them.