That is to cover the 99th percentile. Almost any heating system will be designed to supply enough power, 99% of the time. That 1% us for extreme weather events i.e. -18⁰C or something.
Most of the time a 6 kW unit would do your heatload quite happily. There is a bit added, 2 kW, for heating the hot water.
So seems to be about the right size.