The wood pickets line up perfectly and form a tight space. It is wood on wood - no need for grout here. The reason that it looks like a gap is because of the strong bevel on the pickets. I will upload some other pictures of another project where the wood has no bevel and the fit is even tighter. I prefer to install the pickets, the tiles and grout first and then seal the floor all together.

Here is an image with the 4 pickets joining together.