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These circles must not actually depict coverage areas, as I would expect varying coverages, due to differences in ground conductivity.
Correct. They are based on the radiation patterns of those stations, only, and do not account for earth conductivity.
But at these frequencies and for the omnidirectional, 1 kW daytime power of most of those stations, ground conductivity is not a big factor in the circularity of their 150 µV/m contours — which contours are about the limit of their useful (interference-free) daytime coverage areas.
The graphic below shows the 150 µV/m contours for the 1240 kHz stations in that region based on their radiated powers, and the earth conductivities shown by the “M3” FCC conductivity map for that region of the US.
Stations operating at 1 kW using omni patterns on 1230 and 1250 kHz have very similar effects due to ground conductivity, so those 150 µV/m contours would be rather circular, as well, and can overlap/interfere with the 150 µV/m daytime contours of the nearest stations on 1240 kHz — and vice-versa.