Cold water tap temperature

I was pondering the temperature of the cold water tap!

To get hot water, a domestic hot water system needs to take the cold water up to around 50 degrees C.
– I know there are also considerations to periodically take it above 60 degrees C to prevent the Legionella bacterium growing in the water system.

Looking at the chart below from  http://www.zenexenergy.co.uk/Zenex1/index.php?option=com_content&view=article&id=9&Itemid=5&limitstart=1:

 

Seasonal variations in domestic hot water energy requirements

 

I assumed that the mains water pipes were deep enough to cause less seasonal variation in the output temperatures, between 4°C in the winter and up to 20°C in the summer.

I’m not sure why the graph has a lower summer temperature for domestic hot water (DHW).