Look at it this way...
Let us say there are 10 attackers and 10 defenders.
Now let us say there are five targets:
- Hospital
- Farm field
- Army HQ
- Shopping mall
- Fuel depot
We can assign 2 defenders to each target, but why?
The hospital is clearly a critical target simply for humanitarian reasons. The fuel depot is also a critical target for being a direct contributor to the war effort.
But the Army HQ can be classified as 'collateral damage' because it is essentially an administrative building. In a war, all plans are dispersed, anyway. The building's destruction will not hamper the war one bit.
So out of five targets, we have to defend only two with 10 interceptors, and increased the odds of survival for each.
This is how Iron Dome works. Neither the interception calculators nor the interceptors themselves are perfect, as nothing can be, but the system have more than adequately demonstrated its efficacy to the point that it is desirable throughout the world.