There is quite a difference in the simple model. I'm sure it's not as bad as with BBs, but in the regions I have selected there can be as low as 23% overlap. You can see why when looking at Abercanaid/Troedyrhiw shown here. The smaller the region the worse this effect is (settlements are "odder" shapes than counties?).
The flattening of the circle is due to some sort of datum/projection problem, there have been lots of conversions to get this graphic (this prob due to the cration of the tif, and the space it takes not being square, while the tif is square). It has no effect on the ratio of overlap area to polygon/circle area.
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