Fog depth can easily vary that much or more due to local conditions such as water content in vegetation, wind direction and geography. On other nights we can easily see from Coe that fog is pouring in over the mountains to the south, covering the Peak, while we, at Coe stay dry.
I think it is simply luck of the draw. Win some, lose some.
FWIW, even down the hill from Coe, and when I arrived back home at 9:45, the sky conditions were nothing to get excited about, and I suspect that wile the Peak fared better than Coe, it probably was not the sort of night I would drive there for had I been able to determine what the conditions would be like before going.
Now, if you wanted good observing conditions, you should have been at Houge Park Friday night. Yes, it was cloudy early on...at 9 p.m. I looked outside and saw it was clear... so I grabbed my binos and headed to Houge. There was only one telescope set up, trying to provide views for the nice group from some women's college. Fun time. Friendly crowd. They should have dressed warmer though...