Why do some parts of the West Coast of North America get hot later in the year than others?
I am not an expert, but ...

the pacific ocean has seasonal patterns:



You can think of the water coming from the North being colder, the one from the South warmer.
On top of that, then you have associated rainfall etc. that will affect the climate (it can work both ways, warmer water may mean more rain, then more cloud coverage so less solar radiation so slightly colder days).