Strange steps on a 3D plot

Question

Problematic

Thanks to schrodinger's cat visualizing-a-function-defined-through-a-summation, I adapted his answer to display the value of a bond security that pays a fix amount (Coupon \C) over a certain amount of time. It's the equation in the title of the plot.

Questions

When I try do display the DF function DF(\n,\r)=pow((1+\r),-\n) it's smooth.

Why do I get this strange wavelet when I introduce the sum ?

MWE

\documentclass[tikz,export,fleqn]{standalone}
\usepackage{animate}
\usepackage{pgfplots}
\pgfplotsset{compat=newest}
\newcounter{isum}
\pgfplotsset{summand/.initial=max}
%-------- On définit la somme   ----------------  
\pgfmathdeclarefunction{sum}{2}{%
\begingroup%
\pgfkeys{/pgf/fpu,/pgf/fpu/output format=fixed}%
\edef\myfun{\pgfkeysvalueof{/pgfplots/summand}}%
\pgfmathsetmacro{\mysum}{0}%
\pgfmathsetmacro{\myx}{#2}%
\pgfmathtruncatemacro{\imax}{#1}%
\setcounter{isum}{1}%
\loop
\pgfmathsetmacro{\mysum}{\mysum+\myfun(\value{isum},#2)}%
\ifnum\value{isum}<\imax\relax
\stepcounter{isum}\repeat
\pgfmathparse{\mysum}%
\pgfmathsmuggle\pgfmathresult\endgroup%
}%
\begin{document}

%Different levels of \C, they represent percentage to a dollar
    \pgfplotsinvokeforeach{0.01,0.02,...,0.04}{
        \begin{tikzpicture}[
            declare function={DF(\n,\r)=pow((1+\r),-\n);},
            ]
            \begin{axis}[
            xlabel=Rates,
            ylabel=Time to Maturity,
            zlabel=Bond value,,             
            title={$P=\sum_{k=1}^{n} \frac{#1}{(1+r)^{k}}+\frac{1}{(1+r)^{n}}$},
                    domain=0.0:0.05,    % Interest rates
                    y domain=0:10,  % Maturity
                    view={30}{20},
                ]

\def\C{0.05}
    \addplot3[summand=DF,surf]  {#1*sum(y,x)+ DF(y,x)};
%   \addplot3[summand=DF,surf]  {\C*sum(y,x)+ DF(y,x)};
%   \addplot3[summand=DF,surf]  {DF(y,x)};
            \end{axis}
        \end{tikzpicture}
    }
    \end{document}