(This is possibly made like that on purpose, but I'm still writing this down just in case it's not)

Each of the early buildings take 2 modules as input.
Each module take 4 polyrock nodes to be made, at minimum.

The starting island has 108 polyrock nodes.
108 / 4 = 27
27 isn't an even number.
This means, if you use all polyrocks nodes on the starting island, you have 4 nodes that cannot be paired with another 4 nodes for a furnace input.