How is the position of a mesh island calculated in geometry nodes?

I'm using the distance between cube and cube.001 as the factor for shading.

When I do it without using the positions of the mesh islands, it works as expected.

Then I connected the position values of the mesh islands to the source position socket and the result was a bit weird.

I want to know how mesh islands work and how I can make a setup where the entire island is affected by the shader (properly) and not just the faces.