Answer: It all boils down to the two cities' proximity to the Pacific Ocean, along with wind currents and geographical location
Step-by-step explanation: It is known throughout the whole scientific community that a body of water near a city produces very stable temperatures throughout the year with small dips. When it is summer, instead of San Diego boiling in the heat, the Pacific absorbs most of this heat, making the heat in the city tolerable (almost), this makes the ocean appear cold in the summer.
When it turns to winter, the heat absorbed by the ocean in the summer gets radiated making the cold in San Diego negligible. Applying this same logic to Mount Laguna, which is not near a body of water, it can be understood that it would have more extreme temp. swings.
Along with water, there is wind currents. I don't live in Mount Laguna, but I have to assume from the context you gave me that a cold wind current usually flows there, making it colder than in San Diego, which has a warm current.
The location also matters, if this town is under a valley, canyon, or mountain it could be under a shadow further decreasing the temp. more than San Diego.
Hopefully this helps, and good luck!