Jacket diameter of coaxial cable : how to read those documentations?
There's no jacket on those, just bare outer conductor. Those are straightforward because they are solids. 
The outer dimension is not circular because of the drain wire that's sitting alongside the spiral wound aluminized polyester. The wire is about 0.2mm diameter while the shield is only 1 or 2 layers about 1/10 as thick. So you would expect the dimension across the outside to vary by about 0.2mm, as it does. 
You didn't ask this, but be careful with heat loss calculations in cryogenics, you really have to integrate the heat loss from one temperature to the next rather than just assuming a fixed thermal resistance. Suppliers of cryogenic-qualified cables such as Lakeshore should have that data available.