The article states that there were apparently two missiles. The first one was a non-explosive missile that was merely meant to penetrate a thick cement wall surrounding Arouri's meeting with other Hamas leaders; the second missile was the explosive one that came in afterwards to ensure that there was an explosion inside the building. The first missile seems to be relatively intact.
A third missile appears to have been directed at a car outside the building.
Additionally, there are reports that the cell phones in the area were jammed before the operation, and they came back a short tie afterwards.
The newspaper says, "Israel wanted to send a message beyond the operation that it can use advanced technology to target any target behind any fortifications, in reference to any other Palestinian leaders or parties belonging to Hizballah."
I'm not sure about the concrete wall, unless there was an interior reinforced room, but pictures of the building show that the floor above has an iron grate, which could cause a missile to explode before entering the building. Perhaps there was a similar grate on the floor Arouri was in and the first missile breached it, while the second one blew out everything outward.
