Inconsequent text citations

Question

I want maximum one citation name in my text. Therefore I used the option maxcitenames=1 in the biblatex package. This works for all my references except for two: verdonck2009 and verdonck2011. I am not sure whether this is because the author name is the same or because of some other reason. I have put a MWE example below together with my bibtex references. The result of the MWE example is added in the image at the bottom. As you can see it uses only one citation name, except for verdonck2009 and verdonck2011. Any ideas on how this can be solved?

\documentclass[a4paper,12pt,oneside]{book}
\usepackage[backend=biber, natbib=true, style=authoryear, maxcitenames=1, maxbibnames=99]{biblatex}
\addbibresource{bib/literature.bib}

\begin{document}

In \citet{verdonck2009}, \citet{verdonck2011}, \citet{kaas2008} and \citet{england2002} is stated that...

\printbibliography
\end{document}

@article{verdonck2009,
title={A robustification of the chain-ladder method},
author={Verdonck, T and Van Wouwe, M and Dhaene, J},
journal={North American Actuarial Journal},
volume={13},
number={2},
pages={280--298},
year={2009},
publisher={Taylor \& Francis}
}

@article{england2002,
title={Stochastic claims reserving in general insurance},
author={England, P  and Verrall, R},
journal={British Actuarial Journal},
volume={8},
number={03},
pages={443--518},
year={2002},
publisher={Cambridge Univ Press}
}

@article{verdonck2011,
title={The influence of individual claims on the chain-ladder estimates: Analysis and diagnostic tool},
author={Verdonck, T and Debruyne, M},
journal={Insurance: Mathematics and Economics},
volume={48},
number={1},
pages={85--98},
year={2011},
publisher={Elsevier}
}

@book{kaas2008,
title={Modern actuarial risk theory: using R},
author={Kaas, R and Goovaerts, M and Dhaene, J and Denuit, M},
volume={128},
year={2008},
publisher={Springer Science \& Business Media}
}