TikZ：声明函数与宏：使用 cm

Question

代码：

\documentclass[12pt]{article}
\usepackage{pgfplots}
\usepackage{float}
\pgfplotsset{compat=1.17}
\usepackage{tikz}
\usetikzlibrary{shapes, arrows.meta, automata, positioning, matrix, calc}
\usepackage[RPvoltages, american,siunitx]{circuitikz}
\usepackage[margin=1in]{geometry} 
\begin{document}    
    \begin{figure}[H]\centering
        \begin{tikzpicture} [declare function = {buffHeight = 5; vertDis = 2;}] 
            \def\buffHeight{5};
            \def\vertDis{2};
            \draw(0,0) node[draw, minimum width = 1.25cm, minimum height = \buffHeight] (Samp1){}; 
            \draw(\vertDis,0) node[draw, minimum width = 1.25cm, minimum height = \buffHeight] (Samp2){};
            \draw(0,-6) node[draw, minimum width = 1.25cm, minimum height = buffHeight] (Samp3){};
            \draw(vertDis,-6) node[draw, minimum width = 1.25cm, minimum height = buffHeight] (Samp4){};
        \end{tikzpicture} 
    \end{figure}
\end{document}

结果：

\vertDis和以及vertDis给出了相同的结果。但是当我输入宏和变量版本，即和 时，前者有效，而后者给出错误。为什么宏版本可以与 一起使用，而非宏版本却不行？\buffHeightbuffHeightcm\vertDis cmvertDis cmcm