To put these comments to an end... full credits go to Zarko, who already pointed out all the important facts.
The question is, as far as I understand it, why there can be a white boundary around shapes. The two most common reasons are
- there is an
outer sep
greater than 0 and
- the shape has a boundary path.
This is illustrated by the MWE
\documentclass[tikz,border=3.14mm]{standalone}
\begin{document}
\begin{tikzpicture}
\node[fill,circle,inner sep=3pt,outer sep=0pt] (A){};
\node[fill,circle,inner sep=3pt,outer sep=1pt] (B) at (2,0) {};
\node[fill,circle,inner sep=3pt,draw=white] (C) at (4,0) {};
\foreach \Y in {A,B,C}
{\foreach \X in {-0.5,-0.2,0.2,0.5}
{\draw[thick] ([xshift=\X*1cm,yshift=-5mm]\Y.south) -- (\Y);}}
\end{tikzpicture}
\end{document}

in which B
comes with a nontrivial outer sep
and C
has a white boundary path. Unlike A
, both shapes seem to have a white "halo".
outer sep
greater than zero ...