What I Expected Was?

Use Repeating Cmd Block to summon Skulkers in the same positon, and they teleport to everywhere the distance they can reach. In one word, they can fill a whole range, not as you see the screenshot they don't fill the whole range. They can produce a huge Shulker "block".

What's the Real Situation?

As you can see in the screenshot they're very loose, didn't produce a huge, close "block".

(Cuz I'm not Eng., I tried my best to describe this so there must be sth not very accurate)