Natbib numbers order

Question

I'm having some trouble with BibTeX. The code below makes references in author-year order, but for references with same author and year order is arbitrary. Any solution? Thanks :D

\documentclass{thesis}

\usepackage{indentfirst}
\usepackage{fancyhdr}
\usepackage{graphicx}
\raggedbottom
\usepackage[a4paper,top=4cm,bottom=4cm,left=3cm,right=3cm]{geometry}
\pagestyle{empty}

\usepackage[english]{babel}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage{textcomp}
\usepackage[utf8]{inputenc}
\usepackage{microtype}
\usepackage{enumitem}

\bibliographystyle{siam}
\usepackage[numbers]{natbib}
\usepackage{chapterbib}

\frenchspacing
\usepackage{setspace}
\onehalfspacing




\newcommand{\cit}[1]{``#1''}
\makeatletter
\renewcommand\@biblabel[1]{}
\makeatother


\begin{document}
\cleardoublepage 
\nocite{*}
\renewcommand\bibsection{\subsection*{UNdocs}}
\bibliography{un}
\bibliographystyle{siam}


\end{document}

Entry examples:
@Proceedings{unmik2,
title = {On the Authority of the Interim Administration in Kosovo},
year = {1999},
address = {Pristina},
organization = {UNMIK [United Nations Interim Administration Mission in Kosovo]},
note = {U.N. Doc. UNMIK/REG/1999/1},
}

@Proceedings{unmik3,
title = {On the Appointment and Removal from Office of Judges and Prosecutors},
year = {1999},
address = {Pristina},
organization = {UNMIK [United Nations Interim Administration Mission in Kosovo]},
note = {U.N. Doc. UNMIK/REG/1999/7},
}

Here's two entry in the .bib file order. When I compile they appear in a random order while I'd like the imput order.